265/70/15 is not the same as 31x10.5x15.

265mm/25.4 = 10.4331" wide

10.4331 x .7 (aspect ratio) = 7.3031" sidewall height (approx.)

7.3031 x 2 (sidewalls) = 14.6063"

14.6063" (2 sidewalls) + 15" (rim diameter) = 29.6063 tire diameter.

The formula doesn't allow for minor differences in tire specs, but it'll get you close enough for most calculations. If you need exact, then mark the tire on the floor and mark the floor. Drive straight for 10 revolutions and divide by 10 for distance covered per revolution. Then work back to find the diameter. Sorry, but I don't have scientific symbols set up on this keyboard, but for a hint it includes a slice of "pi". If you take the time to learn the formulae, it'll save you a lot of confusion.

Frank