MPG thread: throttle vs. rpm

[VroomPshhTsi]

I've always wondered what factors affect your mpg the most. In the car I currently drive ('08 honda fit, don't ask), it has a gauge that shows your mpg every couple seconds and for your current tank. Currently it's at 36.7 mpg on the tank (half empty, 220 miles).

My question is this: is miles per gallon affected more by throttle (right foot) or engine rpm? I understand that when u press down on the throttle, the engine opens up for both gas and oxygen. But when you are at high rpm, doesn't your engine need a lot more gas and oxygen even though you aren't necessarily opening the throttle?

On the Fit I drive, the gauge says my mpg drops to single digits only when I open the throttle a decent amount. However, if I downshift and coast (still in gear) with rpm around 4k, it still says I'm getting really good mileage (40+). Obviously when I coast in neutral, the mpg goes up to 200+ cuz I'm going 50+ mph in neutral.

I know aerodynamics and weight are two other huge factors, but I'm asking specifically about throttle and engine rpm, which one hurts your mpg the most.

When I first got my Mustang, mpg wasn't on my mind so I was only getting 18-20 mpg. Now I shift MUCH better and coast on almost every hill (also a lot less WOT runs) and I'm getting 22-23 mpg average per tank. May not sound like much but it is a 15% improvement and adds up.

[stealthy1ss]

I am going to say throttle. Don't quote me on this but I think that when you coast in a high or low rpm there is little to no fuel being used. The reason why I say that is because when I coast my wideband has little dashes across it. The wideband will not pick up an a/f until I get to a point in that gear where the motor starts to get a load put on it.

[Prince Valiant]

My question is this: is miles per gallon affected more by throttle (right foot) or engine rpm?
Well, run a test....run along in 5th gear and try to accelerate. Did fuel mileage go down more closely related to throttle input, or down w/ increasing rpm?

Throttle inputs are FAR more important. My insight shows this in near real-time, as it has a similar, if not the same, fuel mileage gauge that reacts to immediate inputs.

Why is it that despite your engine running in higher rpm's cruising down the road in a lower gear, doesn't your engine burn as much fuel? Because your throttle plate is closed, so only so much air is being allowed past. Your engine is trying to draw in more fuel/air, but the throttle plate restricts this, leading to higher engine vacuum, but not greater fuel consumption.

It's not that a given engine rpm is meaningless...as a general rule, two engines similar size/tune/etc, power similar vehicles, running similar loads (ie, similar speeds/etc), the vehicle running higher gears and thus lower rpm's will get better mileage.

If driving, a good thing to keep an eye on is throttle input needed...if you are going up a slight grade, and can maintain speed w/ less throttle in 4th gear vs 5th gear, then you're at an advantage going to fourth gear...you'll generally get better mileage. If there is no difference in throttle input related to each gear, you'll generally get better mileage in the higher gear.

[That_Guy]

all i know is i get 40-43 mpg with a cold air intake header back exhaust at 70 doing about 2700 rpm and im lovin it!!!

[whistlin six]

If driving, a good thing to keep an eye on is throttle input needed...if you are going up a slight grade, and can maintain speed w/ less throttle in 4th gear vs 5th gear, then you're at an advantage going to fourth gear...you'll generally get better mileage. If there is no difference in throttle input related to each gear, you'll generally get better mileage in the higher gear.

Engine load is the deteriming factor with fuel mileage. ^^^this explains it perfectly. 3000 rpm with no load will burn less fuel than 3000 rpm with a load.
It boils down to manifold vacuum, the higher the vacuum the better the mileage.

[Russ Jerome]

Engine load is the deteriming factor with fuel mileage. ^^^this explains it perfectly. 3000 rpm with no load will burn less fuel than 3000 rpm with a load.
It boils down to manifold vacuum, the higher the vacuum the better the mileage.

Engine load gets my vote as well, then load vs. the rpm furthest from its ideal highest HP/Torq rating followed by its rpm away from its peak volumetric efficency.

Even tho my car gets 23mpg at 68mph it gets 25/26 mpg at 75mph because its closer to the motors sweet spot (convertor already locked) and is under less load even tho its fighting a significant increase in headwind, spent over 2000 miles confirming that a few weeks back.

[VroomPshhTsi]

Good responses.

Prince - that is what has happened. In 5th gear, under load I get crappy (single digit) gas mileage even though engine rpm is under 2k.

I love trying to get the highest mpg per tank (as opposed to highest horsepower like most of us lol). Weren't there some forums out there with people who get ridiculous gas mileage? I thought one was like 28 mpg in a suburban or something.

[BoosTT]

This is how I was told:

Think of it like riding a bike. Is it easier to pedel lightly really fast in a low gear or pedel slow and hard in a high gear?

[Russ Jerome]

Weren't there some forums out there with people who get ridiculous gas mileage? I thought one was like 28 mpg in a suburban or something.

That always interest me, my throttle body Saturn got nearly 40mpg, 4dr, 4 banger and 5 speed at 80 mph but we cant build a brand new domestic SFI crap box to match the milage at the same weight. Likewise my 1997, 280hp Lincoln gets 25mpg at over #4000 pounds but a new 3500# 200hp car gets 18mpg???? Its all a scam!

[wrath]

It's all about adiabatic and volumetric efficiency. And nowadays it also includes drag coefficient just as much as drivetrain losses. Ever notice a two door Cobalt or G5 gets better highway mileage than an Aveo? It's the difference between a Honda Insight-shaped bubble and a sheet of plywood going down the road.

Gloriously underpowered things with small engines and DOHC usually need to "spin to win" because they don't generate enough torque and have so many losses at low RPM that they get poor fuel economy. For example, take the Honda Ridgeline that makes 117 less ponies, weighs 800lbs less, and gets the same fuel economy as my 6.0L truck.

If you have a modern car with throttle-by-wire you don't really know if you have the throttle being delivered being the same as you input. If you have a car that uses CANbus then you can scan it.