My gear ratio stinks (will larger diameter tires help?)

[SilentMatt]

At 70 mph, I'm at 3250 rpm and 3500 rpm at 75. This car definitely wasn't made for highway driving. I'm thinking of getting the tallest tires that will fit to drop the r's down. Not sure how much it'll lower it though. I have some 17" rims that I could throw on it, but seems I've read that everyone loses mileage with bigger wheels. Will I experience the same with taller tires?

[Daox]

Larger diameter wheels weigh more. Larger diameter tires weigh more too, but not as much as larger wheels.

The effect of larger diameter tires on fuel economy is likely to increase highway fuel economy, but lower city fuel economy a little.

[SilentMatt]

I know they'd weigh more, but it's gotta be a pretty insignificant amount to go from a 185/60 to a 70 or 75. Maybe 5-10 lbs a tire?

[Daox]

If you're doing mostly highway travel then it can be worth it.

[SilentMatt]

60+ miles a day.

[slayerized6]

3500 rpm isn't that harmful in the Ecotec engine.

[SilentMatt]

It's pretty harmful to fuel mileage... It's the first car I've ever had with revs over 2500 at cruising speeds. I think it has around a 3.94 diff ratio and 185/75's would effectively put it at a 3.59. Should drop the rpm down to 2900.

[slayerized6]

It's pretty harmful to fuel mileage... It's the first car I've ever had with revs over 2500 at cruising speeds. I think it has around a 3.94 diff ratio and 185/75's would effectively put it at a 3.59. Should drop the rpm down to 2900.

4 cylinder motors are naturally high revving engines. When you have an engine that makes 104hp at 6500rpm, you need to get the revs that high to push the car at 70mph.

Larger and heavier wheels and tires will cause your engine to work that much harder than stock parts to get your car up to and maintain 70mph. If anything, that will cause a decrease in gas mileage. I lifted my Jeep 3 inches and went from 28in stock tires to 32in mud tires. My gas mileage went from 18mpg to 12mpg (Thus the reason I have the Aveo).

[SilentMatt]

I understand a bigger, heavier wheel will take more power to get up to speed, but does it take more power to maintain it?
My engine makes peak hp at 6400 rpm, so would that rpm put the lowest load on the engine? The issue is that the faster the engine spins, the more air gets drawn in and therefore more gas.

If you roll a small diameter wheel and a large diameter wheel on a level surface, the bigger wheel tends to roll further with the same amount of energy. This is probably due to rolling resistance and a flywheel effect. Not sure if this translates into real world, car efficiency though, just trying to brainstorm... The size difference between 185/60 and a 185/75 isn't that drastic. Did the LT(?) that had 15" rims get worse mileage?

I also dealt with Jeep mileage, I had a ZJ that I built a 6.5" lift for and put some 34" Swampers on. That 318 was thirsty to begin with, afterwards it was ridiculous. I was stationed in southern california at the time and gas was $4.50 a gallon...

It's also fact that high rpm equals faster wear on an engine.

[MetroMPG]

It's a little more complicated than just going with larger diameter tires...

1) Rolling resistance changes between different tire types/sizes. This will affect fuel economy a small amount. Could go either way.

2) A taller tire will raise ride height, which will probably increase aerodynamic drag slightly. This will also hurt fuel economy a small amount.

There are online tire size calculators out there that show how changes will affect engine RPM.

And don't forget the larger dia. tires will also cause the odometer/speedometer to under-report distance/speed vs. your current tire. If you don't compensate for that change, your apparent fuel economy will suffer.

FYI, I changed the final drive gearing in my car to reduce cruising RPM by 25%, which is massive (much more than you'll get by changing tire size). My steady speed fuel economy on level ground improved by about 5%: Project 'nerd gear': taller tranny transplant nets +5.2% MPG - MetroMPG.com

Not saying it's not worth doing, but the effect will be fairly small, and you'll want to research tire characteristics and the potential payback before spending the dough.