I was thinking, and I started wondering. How much does rim size change your gas mileage?

Does anyone have any thought, or some hard numbers from before/after?

Mike

Reducing your rotating mass will reduce the amount of energy required to get a car going but after that circumference of the wheel becomes more important. A larger wheel/tire will act as a higher gear ratio and that would lower your engine’s rpm for any given speed giving you an efficiency gain.

[This message has been edited by RACE (edited 05-23-2007).]

It would depend on the weight of the wheel and tire combination. You have to look at the weight of the wheel and the weight of the tire, if both the weight of the wheel and weight of the tire is more, then the total wheel tire combo could weigh significantly more.

Going to a larger circumference combination does not always give better fuel economy. On some vehicles is actually lowers the economy. People on the fuel economy forums have found this out the hard way.

What people forget is what tire you have can also have a impact on fuel economy. If you want better fuel economy you should be looking at LRR (Low Rolling Resistance) tires.

LRR tires are a good start.

Generally, larger wheels lower city milage and raise highway milage. Heavy wheels lower city milage, but have little effect on highway milage.

But dont LRR times have less grip than others?

I conducted a test to see if my new wheel/tire combo would improve fuel economy. The car is an '87 SE with a freshly rebuilt/modded 2.8 V6 and TH125 transmission.

My original wheels were the stock 14x6" alloy with Cooper Sigma 215/60-TR14 tires. According to my bathroom scale, they weighed about 35-36lb each. Total diameter was about 24". Air pressure was set at 32 PSI front / 36 PSI rear. My new wheels are Nippon F-2 17x7" with General Exclaim 215/45-ZR17 tires. According to my bathroom scale, they weigh about 34-35lb each. Total diameter is about 24.5". Air pressure is set at 40 PSI front / 45 PSI rear.

I kept track of the fuel used before switching to the 17" wheels. Over the course of 1780 miles, the car used 98.7 gallons of fuel. That comes out to about 18 MPG. Most of that driving was short hops around town.

Then I installed the aftermarket wheels. Now, 2350 miles later, the car has used 127.9 gallons of fuel. That's an average of about 18.3 MPG. However, during this period I did more highway driving than before.

So in my case, it seems that the new wheels had negligible effect on fuel economy. However, contrary to what was mentioned above, my new Z-rated tires grip a LOT better than the old ones.

You can also raise your tire pressure for better fuel economy.
I keep mine at 45 psi when doing a lot of interstate driving.
You can do some reading at http://www.gassavers.org/ and see what people have tried, what works, and was does not.