RPM question

mac3897 · June 25, 2002

I hope this comes out right as I can barely make sense of it in my own mind.

My question has to do with RPMs and MPG. Lets say that you are going 45 mph and the rpms are steady at 1000 (not actual numbers, just makeing the math easier), and you have the gas pedal pressed only 25% . Now if you keep the pedal in the exact same position and down shift one gear the RPMS will go up to say 1200.

My question ......Is this using more gas then keeping it in the higher gear thus effecting gas mileage, and if so why???

I would guess that the amount of gas being used is the same but i could be wrong.

Hope this makes sense.

CityTruck · June 25, 2002

With the electronics involved in all of today's vehicles, it's probably never a "cut and dry" answer, but I'll throw out a theory:

I would say that the position of the gas pedal is more relevant to the rpms than the gear you're in (obviously, physics poses obvious exceptions... friction, wind resistance, etc; but those factors aside..) The gas pedal being in the same position after a downshift would result in the lower gear now running at the same rpm (1K in the example you provided). In other words, you would be providing the same amount of gas to the engine so I don't think rpm's would increase to 1.2Krpms (once it leveled off). The question then becomes easy to answer because you cover much more distance at the same rpm in a higher gear so it obviously provides better gas mileage. That's my theoretical.

Now the answer based on rpm per gear (3rd vs 4th) is:

3rd (1.0 assuming 4L6E) at 1.2K = 1.2K revolutions

4th (.70 assuming 4L6E) at 1K = 1.43K revolutions

so higher gear at lower rpms still wins

That's my stab at answering the question.

GreenRado · June 25, 2002

I don't know the answer off hand, but I am betting that engine fuel efficiency (as opposed to power efficiency) at 2 different RPMs may play a role also - how much I don't know.

My $0.02.

Shaners · June 25, 2002

I know that when towing a good load, alot of times your best mpg's are when driving in 3 and not "D".

More than once I have been pulling on a steady grade in OD with the torque converter locked-up. I have found that when I pull the truck down into "3" and leave the accelerator pressed down the same distance, the truck will begin to pick up speed.

My theory is that for the load that is on the motor in this case, the truck is more efficient at higher RPM's in 3 vs. OD. That is why the truck will begin to accelerate without giving the motor more fuel. Therefore a lugging motor will use more fuel than one turning more RPM's

SilveradoWV · June 25, 2002

Therefore a lugging motor will use more fuel than one turning more RPM's

Just to throw in a thought, when I had my ZR2 Blazer with the 4.3 I got terrible gas mileage opposed to the 4.8 I have now. It seemed that I was always kicking the tranny down just to keep a constant speed, whereas now with the V8 I stay in the highest gear a lot more and still drive the same road at approximately the same speeds as the V6!

CityTruck · June 25, 2002

I know that when towing a good load, alot of times your best mpg's are when driving in 3 and not "D".

Yeah, that get's into the variables I was talking about.

Clearly if you were getting worse gas mileage in D (OD) then in 3rd, you were actually feeding the engine more fuel than it was able to use (bogging down). The rpm's went up in 3rd but the amount of fuel use was more efficient.

Kind of like if you're doing 50mph in 2nd with the pedal half way down. If you go ahead and floor it, you'll dump more fuel in, but you probably won't accelerate much faster than you would if you pushed the pedal 3/4ths of the way down and used less fuel.

Guessing though. I made the assumption the original question didn't apply to hauling a load.

WEFs99 · June 25, 2002

When you drop down a gear it takes a little load off and that is good. Dropping that gear also raises the RPM so that is bad. My guess is that the bad is just just a skosh more than the good.