Rpm gas mileage

rpm
fuel-economy

#1

In a modern computer controlled fuel injection engine, when the foot is off the gas pedal, does the engine consume more gas if it is in gear versus in neutral (think going down a hill)? I would think not, but I’ve seen it rasied in other forums. Also, without engine lugging, is it better to be in a higher gear but have to press the gas down a bit versus a low gear with the gas pedal depressed less?


#2

Modern F.I. engines cut off fuel flow when you remove your foot from the gas pedal. Shifting to neutral offers no benefit in fuel consumption.

RPM destroys fuel mileage. Reciprocating piston engines efficiency nose-dives with RPM as pumping losses skyrocket. It takes tremendous energy to stop and start those pistons. That’s the drag you feel when you downshift to “brake” going down a steep hill. It’s not “compression braking” there is nothing to compress with the throttle closed…


#3

In most modern cars you will save fuel by staying in gear most of the time. If you want to get really fussy about it you may be able to same a tinny bit more, but it could be at the expense of safety and additional wear on some car parts.

My advice is not to go to neutral when coming up to a stop until the engine is at idle speed or not at all with an automatic. That policy will protect the car and safety and you will be very close the the maximum possible mileage. The best thing you can do to improve mileage is to slow down and anticipate traffic so you stay off the brakes.


#4

Your second question does not describe a realistic condition. If you need gentle acceleration you need only apply light accelerator movement when in the higher gear. Downshifting requires you to depress the pedal even more, not less. You need to increase the engine rpm. Any way you look at it, you get better fuel economy by staying in your highest gear than by downshifting to a lower gear.


#5

… without engine lugging, is it better to be in a higher gear but have to press the gas down a bit versus a low gear with the gas pedal depressed less?

Overall it is better to stay not far under lugging, but each car and each set of conditions vary so it is not a hard fast rule. However the difference is not large enough to worry about if you don’t overdo it. Nothing in the extreme.


#6

“It takes tremendous energy to stop and start those pistons.”

If that were true then it would take a lot of energy just to spin the crank and work the pistons, regardless of throttle position, or even with the head off. Also this “tremendous energy” would end up as heat somewhere, even with no fuel or combustion. And finally, how about the wankel engine and its pumping loses?

“It’s not “compression braking” there is nothing to compress with the throttle closed”

A common misconception. The flaw in this is that a closed throttle is not completely airtight. There is still the idle passage.


#7

In a modern fuel injected engine, the engine will consume more gas when it’s coasting down a hill in neutral because when you’re coasting in gear the computer suts off the fuel flow and lets the car’s momentum turn the engine. If you’re in neutral, the engine has to use a little bit of gas to keep the engine idling. Of course, when you’re in neutral, there’s no engine braking and so theoretically the momentum you save by having it in neutral might save more gas than what it takes to keep the engine idling, but the difference is infintesimal if there is one at all.

With a modern fuel injected engine, so long as you’re at an engine speed that’s still producing power (i.e. not lugging it) fewer RPM’s = fewer explosions= less gas getting used. Some older carburetted cars would use more fuel with more throttle opening at a lower engine speed, simply because of the imperfections of the carb, but modern cars are pretty good at keeping the fuel air mixture constant.


#8

Thanks folks, that was helpful. I’ve concluded from this and other discussions that the best & safest way to get high mpg is to keep the car in gear and count on help from the onboard computer to manage gas flow efficiently.


#9

Todays engines would like to be running at higher RPM’s rather than being lugged. You won’t suffer too much at the gas pump unless you really overdo something. Nobody has a standard answer for your question, but if you don’t lug, you are probably doing well by just pushing the pedal.