I didnt really know what to search for so here I am. I have a 1995 Ford Bronco. From the factory it has 235/75-15 tires and now it has 31x10.50-15 tires. I am trying to figure the true miles traveled to get a true MPG calculation instead of the what the odometer says as it is calibrated for the smaller tire. Right now with the odometer reading off im getting 12.5 mpg which im hoping is way off. Here is a diagram of all the information. Any help is appreciated.



Stock Tire - 235/75R15 Tire 1 - 31x10.50R15
Section Width: 9.25 in Section Width: 10.51 in
Rim Diameter: 15 in Rim Diameter: 15 in

Old Overall Diameter: 28.87 in New Overall Diameter: 30.97 in

Old Sidewall Height: 6.93 in New Sidewall Height: 7.98 in

Old Radius: 14.43 in New Radius: 15.48 in

Old Circumference: 90.69 in New Circumference: 97.29 in

Old Revs per Mile: 720.4 New Revs per Mile: 671.6

Old Actual Speed: 60 mph New Speedometer1: 55.9 mph

Speedometer Difference: - 7.266% too slow
Diameter Difference: - 6.79%

 

Ok if someone knows the answer to this one I’d also love to hear it because my understanding of this has always been this:

If we pretend for sake of argument that at 60 MPH your MPG measuring instrument “which has not been calibrated for the new tire combo”, reads 12.5 MPG and you’ve changed your Mile Revs from 720.4 to 671.6, you’ve reduced your area traveled by 6.816%, and your speedometer is to slow by 7.266% - then I believe your true MPG would be between 11.64 MPG and 11.6532 MPG.

Please feel free to double check - and let me know but I believe those are correct.

 

Multiply the miles recorded on the odometer by .07266 (7.266 percent) and then add that number to the miles recorded to the odometer to get your true miles.

ex. 280 miles x .07266 = 20.34
280. + 20.34 = 300.34 actual miles

 

Thanks for the reply's so far. I like Black Hawks idea better as it shows i am getting 13.4 instead of the 12.5.

 

Yea me too