Higher gear = better mileage?

saturn
ion

#1

The Saturn Ion (at least the particular make I have) is known for having a finicky transmission, but I’ve been able to figure out when and how my car switches gears. Now for local roads, many of them around me have a speed limit of 35 m.p.h… If I accelerate up to 35 and stay there, the RPM hovers at the 2 mark and stays there. However, if I accelerate to around 37 m.p.h., the car will shift up into a higher gear. I then bring it back down to 35 and the car stays in that gear, with the RPM now reading slightly over 1.5. I tend to do this because I assume less RPMs means better mileage in the long run, but since I’m in a higher gear, I’m also assuming the engine has to work harder to maintain that speed at a lower RPM, rather than in the previous gear where the needle stays at the 2 mark.
So that got me wondering - am I saving gas travelling at the same speed but in a higher gear? Or does it not make much of a difference?


#2

Yes you are saving gas in a higher gear for 2 reasons. 1) The engine is not turning as many revolutions per minute (RPM) and 2) The throttle plate is open further because your car doesn’t have as much horsepower at 1.5 (or 1500 RPM) as it does at 2 (or 2000 RPM) That has less “pumping loss” which is a fairly hard to explain technical concept.

But as you may have already found, if you press the gas pedal down just a bit too far, the engine will shift down a gear and get you back to that 2000 RPM again.


#3

I’ve always read that you get the best fuel economy at the lowest speed you can go in the highest gear. That would say that the answer to your question is yes.


#4

Ah, alright! That makes sense to me. Thanks!


#5

I agree with the others, that it’s likely you’re getting better fuel economy by having the vehicle in a lower gear at the same speed.

Something you might consider is that many automatic transmissions choose shift points based on throttle position, and rather than accelerating to select a lower gear, you might try letting off the throttle a little bit to see if it downshifts at a cruising speed of 35 MPH without accelerating.


#6

As long as you don’t lug the engine.


#7

It’s not something that’s real cut and dried, as I learned when experimenting with different gear ratios on a couple of motorcycles. You gear them too high, and mpg actually goes down, even though the engine doesn’t feel like it’s lugging in high.
At the higher rpm gearing, you have higher pumping and friction losses.
At the lower rpm gearing, you have higher thermal losses, there’s more time for heat to escape from the expanding gasses that drive the piston down.
The best rpm is a tradeoff between these two losses and there is a range of rpms where there is little difference, it’s not a narrow and deep notch in the graph.