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9 years ago*

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Bump. Thanks!

Something interesting: In pretty much all modern vehicles (most 1996 or newer, lots of older cars as well), it is more fuel efficient when decelerating to leave the car in gear and take your foot off the gas (at any rpm) than it is to put the car in neutral and coast.

If you put the car in neutral, the engine needs to be supplied with fuel to keep it running at idle or it will stall. If you leave it in gear the computer will shut off the fuel injectors when you take your foot off the gas pedal and very little or no fuel will be used. The momentum of the car will keep the wheels turning which will keep the engine turning. The engine basically turns into an air compressor which is where you get engine braking from.

If you are going down a long hill and put your car in neutral and ride the brakes, you are wasting fuel and brakes. You can instead use engine braking to slow you down to the desired speed while using no fuel and saving your brakes. The higher the rpm your engine is turning while off the gas, the greater the engine braking effect and the quicker the car will slow down. If you are still gaining speed while going down hill and want to slow more, drop to a lower gear. You will be using less fuel in a lower gear at 3500 rpm than you will if you were in neutral at 600 rpm.

Obviously the engine and all moving parts will wear faster at higher rpm so try and keep the engine running at a reasonable rpm. What is reasonable is debatable and will be different on every engine so it is up to you to decide what rpm you feel is safe for the engine. For example, prolonged engine braking at 4000 rpm might be fine in a s2000 that redlines at almost 9000 rpm, but 4000 rpm may not even be obtainable in a diesel engine.

Something interesting 2: It doesn't use any more fuel to accelerate as fast as you want. When you leave a stop light or get on a highway entrance ramp (or any other time you want to increase your speed), it doesn't use any more fuel to put your foot to the floor when accelerating until you reach your desired speed. The only time you waste fuel is when you use your brakes because this means that you wasted fuel accelerating to a higher speed than you needed and had to use your brakes to slow yourself down.

This is more complicated than my first "something interesting" and there are going to be differences between engines and gearing that will change things like optimal shift points. I have not put enough thought or research into that you talk about it, but I think the basic idea of why it works has to do with conservation of energy. Your vehicle weighs a certain amount and it will take a certain amount of fuel to accelerate it from a given speed to a higher speed. Whether you accelerate at a very slow rate for a long time or a very high rate for a short time, it should use about the same amount of fuel. For example if you accelerate from 20 to 60 mph with little throttle and you stay on the gas for 30 seconds or if you do the same at full throttle, but it only takes 5 seconds. You use less fuel over a longer duration or more fuel over a shorter duration, but in the end you use the same amount of fuel.

I don't know if this next part is true, but it sounds logical to me. I have read that it is very slightly more fuel efficient to accelerate at full throttle because when you are at partial throttle, the butterfly valve in the throttle body is not fully opened and creates more resistance on the engine because it is restricting airflow. If this has an affect, it is probably very minimal.

9 years ago*