Origionally my truck came equipped with 255-65-17 tires (30.0" tall). Now I have 285-60-20's (33.41" tall). I know the height of the tire (or circumference) affects the accuracy of the spedometer/odometer. Since my 285-60-20's are taller than the stock tires my spedometer and odometer are off (registering slower and less than actual). I'm not too concerned with the accuracy but what the difference is when calculating my mileage. I would think that I could add a factor to the mileage equasion. Example: If I drove 300 miles and used 22.0 gals. then I would be getting 13.63 MPG. From there, I would hope to multiply 13.63 by 1.?? to obtain my actual mileage. I'm not necessarily looking to correct the error since the odometer increase is less due to the taller tires thus, the miles go on slower. The dealer said my tire size is not in his computer to allow them to reprogram so it appears I looking to the Forum for assistance. OK, who paid attention in math class?
NHRA Fan.

 

You could multiply the miles traveled by 1.113 (the ratio of the revs/mile of the two sizes of tires), then divide by the number of gallons used.

 

Thanks Mange,
I realized my error in referencing the "X" factor at the end of the equasion instead of the only the miles driven miles. Thanks again.