RITmusic2k
05-07-2011, 07:20 PM
I'm posting this in the Performance forum, as I think it's plenty fair to talk about your fuel economy as a component of your car's performance.
A few weeks ago I ran my first 400+ mile tank of gas in this car. That's not a huge stretch by any means, and a lot of it was due to more highway travel than I typically do, but it got me thinking a bit about which of my driving habits resulted in increased fuel economy.
I'm going to be sharing just about every trick I know, but I'll also place an open question to the rest of you in case there's something I haven't covered: what good habits have you developed to stretch out your intervals between fill-ups?
Alright, without further ado, here are
RIKER'S RULES FOR IMPROVED FUEL ECONOMY
1. Changing your state of mind
The best mod for fuel economy can be performed on any car: adjusting the nut behind the wheel. Making a conscious effort to save gas, instead of just... driving... will effect a significant change in the way your car consumes fuel. This was the single biggest factor in improving my economy. How exactly to go about that is described below.
2.Choosing a car with a manual transmission
This might be a contentious claim, but I'm prepared to defend it.
While you may say, "Wait, Kevin... haven't modern automatics actually surpassed manuals in this regard, as verified by current EPA data?", I'll acknowledge that this is true... but I'll point out that the EPA tests are designed to mimic the average (read: unconcerned) driver. We're not talking about meeting our EPA numbers, we're talking about exceeding them. And in my experience, manual transmissions have a few things going for them that autos don't. There's more headroom above those EPA numbers in a stickshift.
It's mainly a matter of having ultimate control over all your variables; manuals always have access to 'neutral' (while possible, it's often unhealthy for a torque-converter automatic to carry speed while the transmission is disengaged), drivers are currently better at predicting road and traffic conditions - allowing us to predict and select the ideal gear earlier than an auto would - and driving a manual properly already requires drivers to maintain a level of awareness of traffic patterns around them.
It's easier to transition between engine-on and engine-off (yes, lots of newer cars implement start/stop tech, but this guide will still be tailored to maximizing economy in the E46... that and, they'll never employ stop/start while the vehicle is moving, wink wink) and it's easier to determine when the car is operating in a fuel-cutoff mode - it's possible to force it to enter a fuel-cutoff mode, in fact.
A lot of the above will be explained in further detail below. For now, suffice to say that there's a certain bit of flexibility in engine manipulation that's exclusive to manual transmissions.
NOTE: I realize that those of you with autos aren't going to turn around and trade your car for a stick shift; most of these techniques work in autos too... they just tend to work better in manuals.
3. Keeping a fuel log
In order to know whether you're saving gas, you need to know how much gas you're using. We've been blessed with instantaneous MPG meters in our cars, and they're a crucial tool in achieving better mileage, and we've also got the onboard computer that can track our economy from tank to tank... but there's a margin of error in the computer and there's no history to refer to.
Keeping a notepad in your car and manually calculating & recording your mileage every fill-up will not only tell you whether your habits are working compared to previous tankfuls, it provides a nice historical record of how healthy your car is. A sudden drop in economy can be an early warning sign, allowing you to address potential problems before they get too significant. Speaking of which...
4. Maintaining your car
A clean air filter. A clean MAF sensor. A fresh fuel filter. Clean injectors. Healthy O2 sensors. Properly inflated tires. Good, fresh engine oil. Quality gasoline. All of these play a role in how well your car turns fuel into forward motion. Keeping up on regular maintenance will ensure that the engine is making the most out of the juice you give it.
-----------------------------------------------------
5. Employing hypermiling techniques
Ahh, here's the part you're probably looking for.
Some of these methods are basic, while others are pretty extreme. Some of them are generally inadvisable and some are flat-out illegal. I'll tell you how to do each of these things, but I'm not telling you to do them. Use whichever techniques feel appropriate for the situation and comfortable to you.
EVERYWHERE:
Drive your car like a bicycle - Never is it more apparent how much work you're putting into motion than when *you* are providing the energy for that motion. If you were on a bike and you saw a red light ahead, you'd never pedal like mad the whole way there and then slam on your brakes just before you reach the intersection. You'd coast until the light changed.
Drive without your brakes - Using your brakes is nothing more than turning gasoline into heat. It's wasting the energy you already spent accelerating your car. Or, if you prefer: if you have to use your brakes now, it means you used too much gas a moment ago.
By being mindful of traffic and other impediments around you (like a traffic jam on the freeway, or the red lights mentioned above), you can minimize the amount of time you spend stopped. Not only does idling waste gas, but resuming from a stop costs way more than rolling up to the intersection/obstruction just as the other cars are about to take off.
Carry as much speed as possible - Seemingly contradictory, this is just to clarify that driving efficiently doesn't necessarily mean driving slowly. Rolling up casually to a stale red light (as mentioned above) is just a means of carrying a higher average speed through the intersection. Carrying as much speed as possible around turns and bends also minimizes the amount of acceleration you have to do once the turn is complete. And it's fun.
Drive with load - This is an interesting one. I could have called it "Avoid cruise control like the plague". People are often told to use cruise control to maximize economy on long trips because it keeps drivers from repeatedly accelerating absent-mindedly or letting their speed creep up over time. The problem is, road trips rarely take place over perfectly level roads, and traversing hills with the cruise on is about the most inefficient thing you can do.
Gravity works against you when you're traveling uphill, and it works for you when you're heading back downhill. Efficiency-minded driving suggests that you'd prefer to accelerate your car when you're getting the extra help from gravity, so it takes less additional fuel to make the same speed gains. Unfortunately, cruise control systems -- even the modern 'adaptive' ones -- are programmed to maintain a set vehicle speed no matter what the conditions. The result is that they dump gas into the engine as you go uphill, effectively trying to accelerate the car when accelerating is hardest... and they back off the gas when accelerating would be easiest. It's exactly backwards.
Instead, you should make sure the car is carrying sufficient speed on level ground as you approach the hill, and then hold your foot at its current throttle position. The car will lose a bit of speed as gravity peels off some of your momentum, but your fuel consumption should be more-or-less unchanged from what it was on level ground. As you crest the hill and begin to descend, continue holding your throttle steady and you'll find yourself accelerating back to your original cruising speed, and your fuel economy should once again remain pretty much consistent the entire way through.
The advanced version of this technique involves you actually backing off the gas a little bit as you go uphill, slowing you moreso than before, but you'll actually be turning an uphill section into a place where you can get a good mileage bump over what you were getting over level ground. We'll come back to this later.
Deceleration Fuel Cutoff - or 'DFCO' as the cool kids call it, is a pretty neat feature of modern fuel-injected vehicles. Whenever you're in gear with your foot off the pedal and your engine rpms are sufficiently above idle speed (a few other criteria must be met as well, but for the most part, these are the only conditions necessary), the car's ECU will shut your injectors completely off. What's happening is your road speed is feeding enough energy back through the car's wheels and through the driveline to keep the engine spinning without the need for any gas. So, if you're coasting toward a stop sign or entering an area with a reduced speed limit, you should stay in gear. This allows your engine to perform a little bit of compression braking to slow you down, and more importantly, you consume absolutely zero fuel until your engine speed drops down to a few hundred rpms over idle. Despite this, however, there's also:
Neutral coasting - if you're going down a steep enough decline, you might find that when you take your foot all the way off the gas, the car does not slow down. You can coast indefinitely, or at least for as long as you've got hill left ahead of you. In this case, it's actually more efficient to coast in neutral than to stay in gear for DFCO. Yes, now your injectors will remain on to keep the engine turning over... but it turns out that if you're not actually trying to slow down, the compression braking that occurs during DFCO robs you of a lot speed and you'd have to spend more gas than you saved to get back to the cruising speed you'd have maintained by coasting out of gear.
Accelerate as gently as possible - This is pretty straightforward. It's not uncontested, though. Some people say they get better mileage by accelerating briskly up to cruising speed, thereby being able to cruise sooner, whereas I find it better to accelerate gently, taking more time to get to cruising speed but wasting less gas on the way there.
An interesting phenomenon reported by people driving cars with smaller engines is that they may find accelerating at heavy load and low speeds (applying wide-open throttle) minimizes the pumping losses of sucking intake air through a cracked-open throttle plate. This evidently yields a measurable difference in consumption. Most larger-engined and all turbocharged cars need not apply.
Drive in the highest gear possible - Just about every car gets its best mileage at the lowest comfortable engine speed in its highest gear. And your car's lowest comfortable engine speed is probably a lot lower than you think it is.
This might scare a few of you. Spend enough time in car forums and you'll run across a post in which someone warns never to 'lug the engine'. Lugging the engine, classically defined, is severe predetonation knocking caused by applying too much load (full throttle, for example) to an engine running at too low a speed (idle, for example). I've never experienced it myself, though I'm told it's horrific. More to the point, you haven't experienced it either.... at least, not in an e46 BMW. Modern fuel-injected cars are immune to lugging, thanks to their knock detection systems. Your car would shut itself down before it would actually lug the engine.
It is possible to strain the engine, though, and you probably have experienced this. If you try to accelerate too hard in too high a gear the car might vibrate unnaturally and make a very laborious loud noise and barely pick up any speed at all... this is what Mr. Forum User is talking about when he hands out his admonishments, but the truth is it's not as unhealthy for the car as he thinks. There is a possible, albeit minimal, risk of starving your bearings since the oil pump would be turning pretty slowly under these circumstances, but it's the sort of behavior that shows up as increased wear over a period of many tens of thousands of miles, *not* catastrophic engine failure.
Okay, now that all that disclaimering is out of the way: drive in the highest gear possible. Accelerating from a stop, I generally shift through 1st-3rd or 4th, then pop straight into 6th. I'm upshifting before 2,000rpms and I'm in 6th by the time I reach 35-40mph. At that speed the car just thrums along quite comfortably at 1,400rpms or so, and it sustains between 35-45mpg.
Just do it. Do it for the children.
ON THE HIGHWAY:
Driving on the highway is already just about the most efficient driving you can do, so there's not much that can be done to improve it... but there is definitely some.
Watch your speed - With a linear increase in speed, aerodynamic drag increases exponentially. That means that on the low side of things, there's only a gradual increase in wind resistance until you hit the knee of the exponential curve. On the far side of that curve, things get really difficult really quickly. No matter what vehicle you're driving, the knee of the aero drag curve falls right around 60mph, so if you don't want more of your engine's power to be used for pushing air instead of moving you, don't go much faster than this.
Use air conditioning - A/C puts an additional load on your motor, but thanks to the facts above, at highway speeds this load is actually lower than the additional drag you'd incur by driving with your windows down. So, really.... don't use air conditioning unless you have to, but use it before you roll the glass down.
Follow somebody - If I call this 'drafting', it's dangerous and against the law. But following another vehicle or driving in a relatively dense pocket of traffic keeps you in a slipstream of air and you'll experience far less drag than you would if you were out in the open. The effect is stronger the closer you are to the leading vehicle, but there is still an observable benefit even if you stay a safe distance behind.
Pulse and Glide - This is a counter-intuitive technique if ever there was one. And once more, I could have called it "Avoid cruise control like the plague". Depending on how you choose to perform it, this technique makes use of several concepts described above: driving with load, DFCO, neutral coasting, and avoiding cruise control like the plague.
You'd think that with all the talk I've been giving about how bad acceleration is for fuel economy, steady-state cruising would be the absolute, bar-none most efficient way to get from point A to point B.
BZZZZZZZZZZZT.
The bizarre truth is that you can do much, much better by spending half the trip with your foot pretty deep on the go-pedal.
Here's the concept: let's say that cruising at a steady 60mph, you can post up a solid 30mpg. Not too shabby. But, imagine now that you start at 55mph and accelerate moderately over a period of 10-15 seconds until you get up to 65mph. During this period of acceleration you're getting about 20mpg. Now, here's where the magic happens. Once you hit 65mph, you take your foot off the gas. The car goes into DFCO and shuts off your injectors. You spend the next 10-15 seconds coasting back down to 55mph, during which time you're effectively getting infinite mpgs. Total time spent = 20-30 seconds. Average speed = 60mph. Average fuel economy (20 for half the period, infinite for the other half) = 40mpg. You maintained the same average speed, but got 33% better fuel mileage by accelerating (pulsing) for the first half and coasting (gliding) for the second half. Simply rinse and repeat, all the way home.
There are three methods of doing this, each with its own level of "intensity" and corresponding level of added fuel savings.
The mild version of this is the one I described in the concept paragraph. In order to do it, you simply use the gas pedal to pulse and take your foot off the gas to glide. In truth it's not quite as ideal as I made it out to be above, either because you might not enter into DFCO, or because you'll decelerate faster because of engine braking and only spend 30% of your time coasting compared to 70% accelerating. Either way, the gains are compromised, though they're still there.
The moderate version involves neutral coasting during the glide portion of the maneuver. By shifting out of gear, you do consume some fuel to keep the engine turning, but you get to coast down a lot more gradually by avoiding engine braking. It's not perfect, but it's a lot closer to that ideal of (50% pulsing / 50% gliding) & (20mpg / infinite mpg).
The extreme version involves shifting into neutral and shutting the engine off completely during the glide. This is risky to say the least, since you immediately lose all steering assist, eventually lose brake boost, and run the risk of locking your steering column if you turn the key back too far. But, during that glide phase you get the best of both worlds from the mild and moderate versions - a long gradual neutral coast, and no fuel consumed. If you are reckless enough to attempt this version, I have two pieces of advice: (1) make sure the ignition switch is returned immediately to the ON position... the same position it rests in after you start the car, and (2) do not use the starter to restart the engine. Instead, you should perform a bump-start. For those unfamiliar, you bump-start the engine by engaging an appropriate gear for the speed you're traveling, and then bringing the clutch up to kick the motor over. NOTE - you are not supposed to dump the clutch; you should briefly and gently approach the friction point for just a moment -- barely enough to let the engine kick back on -- and clutch back in while the engine comes back to life. Then you may rev match and get back into gear for the next pulse.
IN THE CITY:
This is where the most dramatic improvements can be made. Nothing wastes more fuel than accelerating from a stop. This is why highway driving is more efficient than city driving, and most of these techniques are designed to make your city drive a lot more like your highway drive.
DON'T use air conditioning - For the same reasons stated in the highway section, but applied in reverse, if it isn't absolutely necessary to run the A/C you should drive with your windows down to cool off; the additional drag is generally less of a load than engaging the compressor.
Drive the speed limit - These days, most roads in larger communities feature synchronized traffic lights, meaning that once you catch a green, you should catch all of them, as long as you get to the next intersection when the system thinks you should. You maximize the chances of this by driving at the speed limit for a light you happen to catch, driving a little under the speed limit for a light that just turned green, or driving a little faster than the speed limit for a light you just barely made it through. you want to stay as close to the 'middle of the pack' as it maximizes the margin of error if you get to the next light a little early or a little late.
Do this right, and you'll be able to cruise a long way without stopping, likely getting some of the best mileage of your life. A long uninterrupted run on a surface street at 45mph is guaranteed to return better figures than covering the same distance at 65mph on the highway.
Alternately, memorize the traffic light timing on your most-frequently traveled routes. There's always the occasional intersection that just doesn't cooperate with your go-with-the-flow philosophy, so learn whether you need to race up to it or hang back in order to catch the light.
Turn off your engine whenever you can - You won't make every light. You might get pulled over. You might get a craving for some fourthmeal. You might have to wait for the little brat to get out of school. Thanks again to the modern wonder of electronic fuel management, it doesn't require a crapload of gas to start a car these days. Any time you're going to be spending more than ten seconds sitting still, you save gas by turning the engine off and restarting just before you have to resume your travels.
The one time *not* to do this is during the very beginning of your trip. Not only does most of the mechanical wear on an engine take place during a cold start (so you don't want to repeat the process at every red light), but you take longer to get out of the warmup cycle and into closed loop operation. The car will have worse mileage until everything's up to proper operating temperature, so you want to get through that period as soon as possible.
AND FINALLY:
Drive less -
...Um, yeah right. Just kidding, guys!
-----------------------------------------------------
Alright, that's a whole lot of reading in one shot, so I'll cap it off here. There's more that can be shared, but we'll give it some time before I add them in.
Meanwhile, does anyone have anything they'd like to contribute? Comments, questions, agreements, disagreements? Lemme have it.
A few weeks ago I ran my first 400+ mile tank of gas in this car. That's not a huge stretch by any means, and a lot of it was due to more highway travel than I typically do, but it got me thinking a bit about which of my driving habits resulted in increased fuel economy.
I'm going to be sharing just about every trick I know, but I'll also place an open question to the rest of you in case there's something I haven't covered: what good habits have you developed to stretch out your intervals between fill-ups?
Alright, without further ado, here are
RIKER'S RULES FOR IMPROVED FUEL ECONOMY
1. Changing your state of mind
The best mod for fuel economy can be performed on any car: adjusting the nut behind the wheel. Making a conscious effort to save gas, instead of just... driving... will effect a significant change in the way your car consumes fuel. This was the single biggest factor in improving my economy. How exactly to go about that is described below.
2.Choosing a car with a manual transmission
This might be a contentious claim, but I'm prepared to defend it.
While you may say, "Wait, Kevin... haven't modern automatics actually surpassed manuals in this regard, as verified by current EPA data?", I'll acknowledge that this is true... but I'll point out that the EPA tests are designed to mimic the average (read: unconcerned) driver. We're not talking about meeting our EPA numbers, we're talking about exceeding them. And in my experience, manual transmissions have a few things going for them that autos don't. There's more headroom above those EPA numbers in a stickshift.
It's mainly a matter of having ultimate control over all your variables; manuals always have access to 'neutral' (while possible, it's often unhealthy for a torque-converter automatic to carry speed while the transmission is disengaged), drivers are currently better at predicting road and traffic conditions - allowing us to predict and select the ideal gear earlier than an auto would - and driving a manual properly already requires drivers to maintain a level of awareness of traffic patterns around them.
It's easier to transition between engine-on and engine-off (yes, lots of newer cars implement start/stop tech, but this guide will still be tailored to maximizing economy in the E46... that and, they'll never employ stop/start while the vehicle is moving, wink wink) and it's easier to determine when the car is operating in a fuel-cutoff mode - it's possible to force it to enter a fuel-cutoff mode, in fact.
A lot of the above will be explained in further detail below. For now, suffice to say that there's a certain bit of flexibility in engine manipulation that's exclusive to manual transmissions.
NOTE: I realize that those of you with autos aren't going to turn around and trade your car for a stick shift; most of these techniques work in autos too... they just tend to work better in manuals.
3. Keeping a fuel log
In order to know whether you're saving gas, you need to know how much gas you're using. We've been blessed with instantaneous MPG meters in our cars, and they're a crucial tool in achieving better mileage, and we've also got the onboard computer that can track our economy from tank to tank... but there's a margin of error in the computer and there's no history to refer to.
Keeping a notepad in your car and manually calculating & recording your mileage every fill-up will not only tell you whether your habits are working compared to previous tankfuls, it provides a nice historical record of how healthy your car is. A sudden drop in economy can be an early warning sign, allowing you to address potential problems before they get too significant. Speaking of which...
4. Maintaining your car
A clean air filter. A clean MAF sensor. A fresh fuel filter. Clean injectors. Healthy O2 sensors. Properly inflated tires. Good, fresh engine oil. Quality gasoline. All of these play a role in how well your car turns fuel into forward motion. Keeping up on regular maintenance will ensure that the engine is making the most out of the juice you give it.
-----------------------------------------------------
5. Employing hypermiling techniques
Ahh, here's the part you're probably looking for.
Some of these methods are basic, while others are pretty extreme. Some of them are generally inadvisable and some are flat-out illegal. I'll tell you how to do each of these things, but I'm not telling you to do them. Use whichever techniques feel appropriate for the situation and comfortable to you.
EVERYWHERE:
Drive your car like a bicycle - Never is it more apparent how much work you're putting into motion than when *you* are providing the energy for that motion. If you were on a bike and you saw a red light ahead, you'd never pedal like mad the whole way there and then slam on your brakes just before you reach the intersection. You'd coast until the light changed.
Drive without your brakes - Using your brakes is nothing more than turning gasoline into heat. It's wasting the energy you already spent accelerating your car. Or, if you prefer: if you have to use your brakes now, it means you used too much gas a moment ago.
By being mindful of traffic and other impediments around you (like a traffic jam on the freeway, or the red lights mentioned above), you can minimize the amount of time you spend stopped. Not only does idling waste gas, but resuming from a stop costs way more than rolling up to the intersection/obstruction just as the other cars are about to take off.
Carry as much speed as possible - Seemingly contradictory, this is just to clarify that driving efficiently doesn't necessarily mean driving slowly. Rolling up casually to a stale red light (as mentioned above) is just a means of carrying a higher average speed through the intersection. Carrying as much speed as possible around turns and bends also minimizes the amount of acceleration you have to do once the turn is complete. And it's fun.
Drive with load - This is an interesting one. I could have called it "Avoid cruise control like the plague". People are often told to use cruise control to maximize economy on long trips because it keeps drivers from repeatedly accelerating absent-mindedly or letting their speed creep up over time. The problem is, road trips rarely take place over perfectly level roads, and traversing hills with the cruise on is about the most inefficient thing you can do.
Gravity works against you when you're traveling uphill, and it works for you when you're heading back downhill. Efficiency-minded driving suggests that you'd prefer to accelerate your car when you're getting the extra help from gravity, so it takes less additional fuel to make the same speed gains. Unfortunately, cruise control systems -- even the modern 'adaptive' ones -- are programmed to maintain a set vehicle speed no matter what the conditions. The result is that they dump gas into the engine as you go uphill, effectively trying to accelerate the car when accelerating is hardest... and they back off the gas when accelerating would be easiest. It's exactly backwards.
Instead, you should make sure the car is carrying sufficient speed on level ground as you approach the hill, and then hold your foot at its current throttle position. The car will lose a bit of speed as gravity peels off some of your momentum, but your fuel consumption should be more-or-less unchanged from what it was on level ground. As you crest the hill and begin to descend, continue holding your throttle steady and you'll find yourself accelerating back to your original cruising speed, and your fuel economy should once again remain pretty much consistent the entire way through.
The advanced version of this technique involves you actually backing off the gas a little bit as you go uphill, slowing you moreso than before, but you'll actually be turning an uphill section into a place where you can get a good mileage bump over what you were getting over level ground. We'll come back to this later.
Deceleration Fuel Cutoff - or 'DFCO' as the cool kids call it, is a pretty neat feature of modern fuel-injected vehicles. Whenever you're in gear with your foot off the pedal and your engine rpms are sufficiently above idle speed (a few other criteria must be met as well, but for the most part, these are the only conditions necessary), the car's ECU will shut your injectors completely off. What's happening is your road speed is feeding enough energy back through the car's wheels and through the driveline to keep the engine spinning without the need for any gas. So, if you're coasting toward a stop sign or entering an area with a reduced speed limit, you should stay in gear. This allows your engine to perform a little bit of compression braking to slow you down, and more importantly, you consume absolutely zero fuel until your engine speed drops down to a few hundred rpms over idle. Despite this, however, there's also:
Neutral coasting - if you're going down a steep enough decline, you might find that when you take your foot all the way off the gas, the car does not slow down. You can coast indefinitely, or at least for as long as you've got hill left ahead of you. In this case, it's actually more efficient to coast in neutral than to stay in gear for DFCO. Yes, now your injectors will remain on to keep the engine turning over... but it turns out that if you're not actually trying to slow down, the compression braking that occurs during DFCO robs you of a lot speed and you'd have to spend more gas than you saved to get back to the cruising speed you'd have maintained by coasting out of gear.
Accelerate as gently as possible - This is pretty straightforward. It's not uncontested, though. Some people say they get better mileage by accelerating briskly up to cruising speed, thereby being able to cruise sooner, whereas I find it better to accelerate gently, taking more time to get to cruising speed but wasting less gas on the way there.
An interesting phenomenon reported by people driving cars with smaller engines is that they may find accelerating at heavy load and low speeds (applying wide-open throttle) minimizes the pumping losses of sucking intake air through a cracked-open throttle plate. This evidently yields a measurable difference in consumption. Most larger-engined and all turbocharged cars need not apply.
Drive in the highest gear possible - Just about every car gets its best mileage at the lowest comfortable engine speed in its highest gear. And your car's lowest comfortable engine speed is probably a lot lower than you think it is.
This might scare a few of you. Spend enough time in car forums and you'll run across a post in which someone warns never to 'lug the engine'. Lugging the engine, classically defined, is severe predetonation knocking caused by applying too much load (full throttle, for example) to an engine running at too low a speed (idle, for example). I've never experienced it myself, though I'm told it's horrific. More to the point, you haven't experienced it either.... at least, not in an e46 BMW. Modern fuel-injected cars are immune to lugging, thanks to their knock detection systems. Your car would shut itself down before it would actually lug the engine.
It is possible to strain the engine, though, and you probably have experienced this. If you try to accelerate too hard in too high a gear the car might vibrate unnaturally and make a very laborious loud noise and barely pick up any speed at all... this is what Mr. Forum User is talking about when he hands out his admonishments, but the truth is it's not as unhealthy for the car as he thinks. There is a possible, albeit minimal, risk of starving your bearings since the oil pump would be turning pretty slowly under these circumstances, but it's the sort of behavior that shows up as increased wear over a period of many tens of thousands of miles, *not* catastrophic engine failure.
Okay, now that all that disclaimering is out of the way: drive in the highest gear possible. Accelerating from a stop, I generally shift through 1st-3rd or 4th, then pop straight into 6th. I'm upshifting before 2,000rpms and I'm in 6th by the time I reach 35-40mph. At that speed the car just thrums along quite comfortably at 1,400rpms or so, and it sustains between 35-45mpg.
Just do it. Do it for the children.
ON THE HIGHWAY:
Driving on the highway is already just about the most efficient driving you can do, so there's not much that can be done to improve it... but there is definitely some.
Watch your speed - With a linear increase in speed, aerodynamic drag increases exponentially. That means that on the low side of things, there's only a gradual increase in wind resistance until you hit the knee of the exponential curve. On the far side of that curve, things get really difficult really quickly. No matter what vehicle you're driving, the knee of the aero drag curve falls right around 60mph, so if you don't want more of your engine's power to be used for pushing air instead of moving you, don't go much faster than this.
Use air conditioning - A/C puts an additional load on your motor, but thanks to the facts above, at highway speeds this load is actually lower than the additional drag you'd incur by driving with your windows down. So, really.... don't use air conditioning unless you have to, but use it before you roll the glass down.
Follow somebody - If I call this 'drafting', it's dangerous and against the law. But following another vehicle or driving in a relatively dense pocket of traffic keeps you in a slipstream of air and you'll experience far less drag than you would if you were out in the open. The effect is stronger the closer you are to the leading vehicle, but there is still an observable benefit even if you stay a safe distance behind.
Pulse and Glide - This is a counter-intuitive technique if ever there was one. And once more, I could have called it "Avoid cruise control like the plague". Depending on how you choose to perform it, this technique makes use of several concepts described above: driving with load, DFCO, neutral coasting, and avoiding cruise control like the plague.
You'd think that with all the talk I've been giving about how bad acceleration is for fuel economy, steady-state cruising would be the absolute, bar-none most efficient way to get from point A to point B.
BZZZZZZZZZZZT.
The bizarre truth is that you can do much, much better by spending half the trip with your foot pretty deep on the go-pedal.
Here's the concept: let's say that cruising at a steady 60mph, you can post up a solid 30mpg. Not too shabby. But, imagine now that you start at 55mph and accelerate moderately over a period of 10-15 seconds until you get up to 65mph. During this period of acceleration you're getting about 20mpg. Now, here's where the magic happens. Once you hit 65mph, you take your foot off the gas. The car goes into DFCO and shuts off your injectors. You spend the next 10-15 seconds coasting back down to 55mph, during which time you're effectively getting infinite mpgs. Total time spent = 20-30 seconds. Average speed = 60mph. Average fuel economy (20 for half the period, infinite for the other half) = 40mpg. You maintained the same average speed, but got 33% better fuel mileage by accelerating (pulsing) for the first half and coasting (gliding) for the second half. Simply rinse and repeat, all the way home.
There are three methods of doing this, each with its own level of "intensity" and corresponding level of added fuel savings.
The mild version of this is the one I described in the concept paragraph. In order to do it, you simply use the gas pedal to pulse and take your foot off the gas to glide. In truth it's not quite as ideal as I made it out to be above, either because you might not enter into DFCO, or because you'll decelerate faster because of engine braking and only spend 30% of your time coasting compared to 70% accelerating. Either way, the gains are compromised, though they're still there.
The moderate version involves neutral coasting during the glide portion of the maneuver. By shifting out of gear, you do consume some fuel to keep the engine turning, but you get to coast down a lot more gradually by avoiding engine braking. It's not perfect, but it's a lot closer to that ideal of (50% pulsing / 50% gliding) & (20mpg / infinite mpg).
The extreme version involves shifting into neutral and shutting the engine off completely during the glide. This is risky to say the least, since you immediately lose all steering assist, eventually lose brake boost, and run the risk of locking your steering column if you turn the key back too far. But, during that glide phase you get the best of both worlds from the mild and moderate versions - a long gradual neutral coast, and no fuel consumed. If you are reckless enough to attempt this version, I have two pieces of advice: (1) make sure the ignition switch is returned immediately to the ON position... the same position it rests in after you start the car, and (2) do not use the starter to restart the engine. Instead, you should perform a bump-start. For those unfamiliar, you bump-start the engine by engaging an appropriate gear for the speed you're traveling, and then bringing the clutch up to kick the motor over. NOTE - you are not supposed to dump the clutch; you should briefly and gently approach the friction point for just a moment -- barely enough to let the engine kick back on -- and clutch back in while the engine comes back to life. Then you may rev match and get back into gear for the next pulse.
IN THE CITY:
This is where the most dramatic improvements can be made. Nothing wastes more fuel than accelerating from a stop. This is why highway driving is more efficient than city driving, and most of these techniques are designed to make your city drive a lot more like your highway drive.
DON'T use air conditioning - For the same reasons stated in the highway section, but applied in reverse, if it isn't absolutely necessary to run the A/C you should drive with your windows down to cool off; the additional drag is generally less of a load than engaging the compressor.
Drive the speed limit - These days, most roads in larger communities feature synchronized traffic lights, meaning that once you catch a green, you should catch all of them, as long as you get to the next intersection when the system thinks you should. You maximize the chances of this by driving at the speed limit for a light you happen to catch, driving a little under the speed limit for a light that just turned green, or driving a little faster than the speed limit for a light you just barely made it through. you want to stay as close to the 'middle of the pack' as it maximizes the margin of error if you get to the next light a little early or a little late.
Do this right, and you'll be able to cruise a long way without stopping, likely getting some of the best mileage of your life. A long uninterrupted run on a surface street at 45mph is guaranteed to return better figures than covering the same distance at 65mph on the highway.
Alternately, memorize the traffic light timing on your most-frequently traveled routes. There's always the occasional intersection that just doesn't cooperate with your go-with-the-flow philosophy, so learn whether you need to race up to it or hang back in order to catch the light.
Turn off your engine whenever you can - You won't make every light. You might get pulled over. You might get a craving for some fourthmeal. You might have to wait for the little brat to get out of school. Thanks again to the modern wonder of electronic fuel management, it doesn't require a crapload of gas to start a car these days. Any time you're going to be spending more than ten seconds sitting still, you save gas by turning the engine off and restarting just before you have to resume your travels.
The one time *not* to do this is during the very beginning of your trip. Not only does most of the mechanical wear on an engine take place during a cold start (so you don't want to repeat the process at every red light), but you take longer to get out of the warmup cycle and into closed loop operation. The car will have worse mileage until everything's up to proper operating temperature, so you want to get through that period as soon as possible.
AND FINALLY:
Drive less -
...Um, yeah right. Just kidding, guys!
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Alright, that's a whole lot of reading in one shot, so I'll cap it off here. There's more that can be shared, but we'll give it some time before I add them in.
Meanwhile, does anyone have anything they'd like to contribute? Comments, questions, agreements, disagreements? Lemme have it.