Tire Size vs Gas Mileage???

[Richard B.]

Must have taken my stupid pill, but can't figure this out. I went from 28 inch diameter tires, to 29 inch diameter ( about 3% more circumference ) and gas mileage dropped from about 17.5 to 16.5. I checked the speedo against highway mileage markers and am doing 62 with the cruise control pegged at 60. I assumed the speedo was proportional to the odometer.

I figured the odo was off by 3-4%, but that doesn't account to the 10-12% drop in mileage. Is the difference due to rolling resistance??? or am I missing something?

Once again, Many Thanks for the help.

Regards,

Rich.....

[Michael W.]

Re: Tire Size vs Gas Mileage???

I get at least a 1 MPG variation from tank to tank and season to season without making any changes to the vehicle at all. 1 MPG decrease from 17.5 to 16.5 is not 10-12% decrease, it's approx. 5.6% if that helps.

If you have not recalibrated your odometer since the tire change, don't forget that the vehicle is now traveling further per tire revolution (approx 3.14 inches or 3.4%)than it used to, so comparing pre/post odo readings to fuel consumed is almost meaningless.

[Matt Purdy]

Re: Tire Size vs Gas Mileage???

Rolling Resistance will affect mileage, but I bet that the issue is this:

The tire diameter went up, so your cruise RPM's went down, so the engine is "lugging" a little more trying to maintain speed. You are no longer in the "sweet spot" of your torque range.

[Wayne K.]

Re: Tire Size vs Gas Mileage???

Richard ,

Your mileage change sounds about right. If your mileage reading is off 4% and you went 165 miles on 10 gal. your mileage is 16.5. You've actually gone 171.6 miles because of the 4% error therefore your true mpg is 17.16. You've changed your gear ratio so you may be using more gas during acceleration than you did before. Perhaps there is someone with more or better thinking but I think your numbers are in the ballpark.

Wayne

[Richard B.]

Tire Size vs Gas Mileage???

Thanks for all the help!!! I only worked the numbers out in my head. Didn't even consider the change in "gear ratio" for acceleration, cruise at a higher speed due to speedo error, and possible increased air drag. I'll get used to the numbers, and monitor them for changes.

First snow this morning in Chicago, so no more fun until spring. :-(

Regards,

Rich.....