[billt]

Here's the deal. I need some help with the math involved in this. My truck, (1991 F-150), came equipped with P-235 X 15 tires from the factory. I don't know the rolling circumference of that tire. After they were worn, (around 22,000 miles), I replaced them with 31 X 10.5 X 15 tires. Obviously these tires have a larger rolling circumference that has effected both odometer and speedometer accuracy. I have had that size tire on the truck ever since.

My truck now has 152,000 miles on it. Which means for the last 130,000 miles it has registered fewer miles, and a lower indicated MPH because of the larger diameter tires. Is there a way to calculate the difference, so I know with some degree of accuracy, how many actual miles my truck has on it?

 

[TommyGunn]

circumference = pi X diameter. Pi = 3.141592654.

If you have 15" diameter tires then the circumference is 47.124 inches.
You need to know the diameter to get the answer.

Then figure the difference between the old tires and new.

 

[flipper35]

P235 x what x 15?

235 is the width and 15 is the wheel diameter, but the profile is missing. if it is a 235/70 then the sidewall height is 70% (165mm/6.5") of the width (235mm/9.25").

 

[Tim Burke]

Do you have a GPS? Compare GPS measured speed (GMS) to speedometer speed (SMS) to find your answer.

(GMS/SMS) * measured mileage = actual mileage

 

[csmkersh]

Calculating Tire Dimensions

Tire Tech Information - Calculating Approximate Tire Dimensions

 

[Diamondback]

And get your speedo and odometer regeared and corrected.

 

[billt]

p235 x what x 15?

235 is the width and 15 is the wheel diameter, but the profile is missing. If it is a 235/70 then the sidewall height is 70% (165mm/6.5") of the width (235mm/9.25").

235 x 75 x r15