How to drive in a manner that increases your fuel economy

You'd be amazed by how difficult it is to find information on the internet to support this node (though I finally did, thanks http://www.epa.gov/otaq/17-tips.htm). Whenever we talk about conserving gasoline, we talk about avoiding or minimizing driving as an activity altogether. In other words, don't drive if you don't really have to, and carpool or ride the bus whenever necessary. Nobody seems to be concerned with how we drive, they're more worried about how often we drive. Here are some basic tips that can go a long way towards improving your fuel economy:

• Whenever possible, avoid coming to a complete stop. Don't rush towards stoplights, instead slow down and coast (unless traffic is bad). All it takes is a little depth perception in most cases. When your car is at a complete stop (idling), your fuel economy is exactly 0 miles per gallon. The act of getting your car to start from a complete stop has a dramatic impact on your fuel consumption, which is why "highway" fuel economy is always better (despite the higher speeds) than "city." On the other hand, don't be annoying about it. In bad traffic, this can really piss people off. Blowing stop signs might not be a good idea. But the idea is to keep your car moving as much as you can. Even if you're only rolling at 5 mph before the light changes to green, you're still saving gas by not stopping entirely.

• Stay off of your freakin' brakes in general, whether in the city or on the highway! Nothing annoys me more than a brake-happy fool who constantly taps their brakes and then re-accelerates, oscillating between 55 and 65 mph depending on the ebb and flow of their cell phone conversation. How counterproductive is that? Try to use your brakes and accelerator as little as possible (see the end for more on this). Stay at a constant speed, don't fall below your desired speed or you'll have to waste energy speeding back up. Use cruise control on long drives (assuming you have it, that is).

• Minimize the amount of weight in your car. The more shit you're lugging around, the more energy you're wasting. It's a simple fact that it takes less effort to move less weight. It's probably not a good idea to dump your spare tire, though.

• Keep your car in good working order. It's a good idea to change your PCV valve, crankcase filter, and air filter at least every 12,000-15,000 miles. Change your oil at least every 4,000-5,000 (the 3,000 figure is more of a conspiracy to get you to buy more oil than anything else) miles, more frequently depending on how hard you drive. Change your spark plugs! Get tune-ups! A bad sensor can ruin your fuel economy. The better your engine works, the more efficiently it burns gas. In many cases, a faulty brake system can drastically decrease your economy. As brakes begin to deteriorate, they often have difficulty completely disengaging, meaning you must drive with un-needed, added friction. Drum brakes can be especially prone to this, as the springs that engage/disengage braking can become old and weak.

• Don't drive like a bat out of hell. In many cases, accelerating quickly is entirely pointless. Granted, it's fun, but you'll do much better to gradually accelerate.

• Avoid speeding. Yes, it's fun, but supposedly you can improve your gas mileage by as much as 15% by going 55 mph instead of 65. The more often your engine is revved high, the worse your economy will be. Of course, if you have enough torque, you might be able to do 65 mph at 1500-2,000 rpm.

• Check your tire pressure often. If your tires are abnormally low, your engine must work harder to travel an equivalent distance, thus reducing your fuel economy.

• Don't use your air conditioner unless you really need to. The air conditioner has a compressor that is powered by a belt right from your engine, so it basically saps horsepower and gas mileage.

• If you drive an automatic, use overdrive (if you have it). If you drive a stick, try to stay in a higher gear whenever possible. The less your engine is revved between shifts, the better. Of course, you can overdo this and cause bad carbon deposits to build up in your engine. Find a comfortable medium between peeling out and stalling out.

• Reduce drag whenever possible. If you're driving a 1984 Honda Civic, chances are you will never reach speeds high enough to necessitate the added downforce a spoiler provides. Instead, it will serve as a source of added drag during normal driving conditions. Also, it adds weight. Whee! Cutting off your side rear-view mirrors can reduce your drag by as much as 20%! Of course, it's most likely highly illegal, although many cars don't have one on the passenger side anyway ...

The most important factor in your fuel economy is your choice of car; the second would probably be how you choose to drive it. I am seriously considering a Honda Insight myself, although I am kind of afraid of tiny sardine-can cars that weigh less than 2,000 lbs. On the other hand, it gets incredible gas mileage, and it lets you know how you're doing at any given moment (which my girlfriend's dad's Ford Explorer is also capable of, although it has to be less gratifying to see "13 mpg" 80% of the time).

Wintersweet had the following additional advice:

• If you're going over 45 mph and it's hot, you are actually better off using A/C because the efficiency loss due to drag caused by open windows exceeds the efficiency loss caused by powering the A/C compressor. Makes sense, since open windows DO create a ton of drag.

• By overinflating your tires by about 2 psi, you can improve your efficiency by as much as 10%. This is safe, she says, because manufacturer recommendations are for a softer ride, not necessarily a safer one. So you'll gain up to 10% and might have a slightly rougher ride.

sleeping wolf had this to say:

"I actually recall reading about a study by Mercedes-Benz that says that the best way to cut fuel economy is to stomp on the gas when you need to (and then coast)."

If you allow yourself to coast too much (to where you fall below your desired speed) you will have to waste more gas by depressing the accelerator. So the best strategy would be to gradually reach your desired speed (no stomping) and then to maintain speed by lightly pressing the accelerator whenever necessary (basically what cruise control does). I didn't mean to say "stop braking and accelerating as much as possible" in the sense that you should fall below speed; rather maintain a constant speed as smoothly as possible.

Finally, a tip from bigmouth strikes:

• "Another thing to save fuel is to always use engine braking. Doing so, the car's momentum drives the engine and NO fuel is consumed. Better than idling, thus. Can be used on automatics as well as stick shifts."

Actually the best way to decrease fuel consumption is sometimes called 'squirt and coast'.

To do this you accelerate using moderate to firm acceleration (choose gears to give about 1/3-1/2 max engine revs or whatever is most efficient for your engine for accelerating) up to a target speed (a speed like 56 mph is OK, but the lower the better due to air drag; 30 mph is better ;-) ). You should try to get about 20-25 miles per gallon while accelerating depending on the engine.

When you reach your target speed you drop the clutch (or put the car into neutral), and coast with the engine idling. You should easily get more than 50 mpg whilst you are coasting, even on a real gas guzzler, probably well over a hundred mpg on an average car.

You should aim to coast for a few times longer than you accelerate for. When you've lost say, 10-15 mph, accelerate back up to your target speed. Lather, rinse, repeat.

Doing this can more than double your miles per gallon, on even an inefficient car; it's by far the most significant thing you can do.

The problem with it is the coasting phase. It will really annoy the person behind you and coasting that way is often illegal and it can be dangerous.

Still, it is very, very effective. Some essentially unmodified road cars with normal engine sizes can achieve a average of well over 120 mpg this way (although the experts often add an ignition kill switch to temporarily kill the engine whilst coasting to achieve this, and then bump start it again; NOT recommended if your car has power assist on the brakes or steering!)

I've used this only once; during a petrol tanker strike in the UK, and there were no almost no cars on the road; it definitely works, my gas guzzling car went much, much further; at least twice as far on the same fuel, probably more (difficult to be more precise when you can't fill up with petrol).

In fact, lordaych's node says that going below your optimum speed wastes fuel. That's not actually true, if you rolled to a complete stop before restarting your engine, you would actually go further, it's just that you gain less if you go much below 30-40 or so miles per hour (particularly if your engine is idling rather than switched off), and it takes you far longer to get to where you want to go. However it *would* be true if you weren't idling your engine- internal combustion engines aren't very efficient at doing that; my car indicates about 50 mpg rolling at 22 miles per hour with the engine out of gear and idling. This implies it gives 100 mpg rolling at 44 miles per hour and so on.

The squirt and coast technique is used for distance racing; internal combustion engines can do >3000 miles on a single gallon, if you keep your top speed under 30mph and have a tremendously lightweight car with good aerodynamics and low friction wheels.

Blondino messaged me that: "the most efficient way of increasing fuel economy is naturally to walk or cycle if you can rather than take the car..." Thanks Blondino, that's deep thinking! I certainly agree that nobody should do any unnatural walking or cycling. However, using a car cannot always be avoided and then these techniques can help.

Most of the above information is correct. To achieve maximum fuel efficiency, all of the above will work. But for realistic high fuel efficiency - IE, not coasting to a stop from 60mph on the freeway - things require a different touch.

For instance, the highest gear is not always the best. If you are at 35mph and in 5th gear (Around 1800RPM in the average car), you will actually be wasting more gas than if you were in 4th or even 3rd. Ever car has a certain range where it gets the best fuel economy, and it is usually right before the secondary air system opens up (on newer FI secondary intake runnersor carburated secondary "power" venturis cars with more than one barrel). Most cars have secondaries that open around 2800-3500 RPM, and this lets more air, and thus, fuel into the engine. The best RPM to be at to achieve best fuel economy is at right below this point.

Here's an important thought to consider when figuring out how to maximize your economy: some vehicles are most efficient at speeds above the speed limit. This is a highly counterintuitive concept but it does make sense, and here's why. Your engine is most efficient within a certain range of RPMs. Usually this is somewhere in the middle of the RPM range, often a bit above that. While at very high RPMs you do lose quite a bit of efficiency due to friction, this is not such an issue at nominal running speed. Meanwhile, your transmission trades speed (RPMs) for torque in all gears except your overdrive gear, in which it trades torque for speed (revolutions per minute, measured at the tires.) Thus, provided your car is appropriately aerodynamic, your best efficiency in miles per gallon (barring the squirt-throttle technique) can often be found in the middle of your engine's powerband, and in overdrive.

In my 1989 Nissan 240SX this is at about 80 miles per hour in fifth gear - this is where I get my best freeway mileage (approximately 30 mpg.) In my 1981 Mercedes 300SD Turbo-Diesel, it's also at about 80 MPH. Note that both of these cars are geared fairly low, because neither of them has much power compared to other vehicles in their class; this means that they are closer to their maximum engine RPM rating at cruising speed than many other cars.

Do not consider this an incitement to speed, only food for thought.

By the way, engine braking doesn't necessarily help your fuel consumption any. To understand why this is, you need to understand how the engine decides to deliver fuel. On any vehicle with an oxygen sensor (aka O2 sensor) the driver decides how much air to admit to the engine (if that - more on this later) and the computer decides how much fuel to deliver. On carbureted (yuck) vehicles, this is accomplished by the mixing control, which could be a solenoid but is usually a motor that continually opens and closes a port. On fuel-injected vehicles, this is controlled by changing the fuel pressure, pulse width, or just the duty cycle of the fuel injectors.

Either way, the PCM (powertrain control module - used to be called ECU, or engine control unit) is constantly monitoring the oxygen sensor. The O2 sensor, when heated, generates electricity because oxygen ions are attracted to it, and pass across it, inducing a current. More voltage means a leaner mixture, which is to say that it has more oxygen in it because less of the oxygen has been burned. Less voltage means a richer mixture, which means more of the oxygen has been burned. When the PCM detects that the mixture is lean, it adds fuel; when it is rich, it reduces it. This is going on continually while you drive, once the vehicle heats up (generally, as detected by the coolant temperature sensor.) If you hook the O2 sensor output up to a digital oscilloscope you can watch it draw a neat litle wavy line on your screen, rising and falling up to a few times a second.

Whether you are in overdrive or in second gear, when you let up on the accelerator pedal, the throttle butterfly valve (a round valve which closes off the air intake) is closed. This is where I clarify the point raised earlier about the user determining how much air enters the engine; in modern vehicles you may not even get to do that much. On vehicles which use GM's NorthStar engines, they use throttle-by-wire, which means that the only thing connected to the pedal is a potentiometer (the same kind of thing used for a volume knob) and the butterfly is controlled by a servo, as is fuel delivery. Even on some cars without throttle-by-wire, there is a secondary butterfly, usually located at the intake ports on the cylinder heads, which the PCM can use to decrease the air intake.

If you are engine braking, then the engine has to run faster than it does when you are not engine braking. Except in a few vehicles, this does not result in fuel and spark being shut off. Generally speaking, each revolution with the throttle closed uses a fairly finite quantity of fuel. As a result, engine braking will cause most vehicles to consume more fuel. This doesn't make engine braking useless; it saves your brakes. Most importantly, if you're not using your brakes they don't heat up, so engine braking continues to be an absolute necessity when descending long, steep hills. And, ultimately, it isn't all that much more. But the point is that the PCM is trying to run as close to a stoichiometric burn ratio as possible (about 14.7:1 ratio of air to fuel by mass at sea level with nominal temperature and barometric pressure) and so with the throttle plate closed, that's going to be more fuel per second when the engine turns at a higher number of RPMs.

Some vehicles DO cut fuel during engine braking; this is apparently common in modern BMWs. If you think your car does it, here's a quick test that you can do in vehicles with a manual transmission: get yourself on a nice straight, flat piece of road with no one else around, and get up to speed. Now, let out the gas pedal - you are now engine braking. Now, turn off the ignition. This will stop your power steering, power brakes, et cetera, so be sure to do this in a safe location, and don't blame me if you get killed! If the car's engine braking slows you down more with the ignition off than with it on, that probably means that your car is not doing a fuel cut. This is probably not a guaranteed test, because there's any number of tricks that can be done to change the way the car behaves, and on a modern vehicle with an automatic transmission you actually have no control over the transmission itself. Instead of using a manually-actuated valve body the shifter is attached only to a switch and the valves are actuated by solenoids controlled by a small computer, which is usually separate from but still talks to the PCM.

Now a couple quick notes on other things you can do to improve mileage: There are a number of additives which claim to do this, including acetone which can be purchased at any hardware store. Don't run more than 5% acetone, and don't come to me if it does something bad to your car, although the worst thing it should do is maybe damage your seals. That's bad enough, so if you're not sure, don't do it. Also, while most aerodynamics modifications are targeted at increasing downforce and thus drag, not all of them fit into this category. In particular, kits for third and fourth generation corvettes sometimes include an underpan which reduces drag on the underside by reducing turbulence, which is done in order to reduce lift (corvettes of these generations produce a great deal of front-end lift at high speeds, which makes the car very dangerous to drive fast without modification.) And along the lines of the note on removing your mirrors above, there are "aero mirrors" which are simply smaller mirrors with which you can replace your factory mirrors. They are available for basically all sports cars, and anything that is commonly modified (like the Honda Accord.)