Ok...

Discovery... None of these take into account tire buldge (IE because it has weight on it)...

So I have 235 45 17 so to calc radius (I'll work in metric so it ends in km/hr)

45% of 235 = 105.75mm profile

17" = 431.8mm

So calc radius of my wheels are half of mag + profile = 431.8/2 + 105.75 = 321.65mm

Upon measuring (for whatever factors cause the difference) I measure 298mm

So that is 1 inch in radius and 2 in diameter that these equations are out...

If you use this formula to get km/hr (where * is times, / is divide, r is measured loaded radius from ground to wheel centre, R1 is transmission ratio, R2 final (diff) ratio, and RPM is your desired RPM, and remember order of operations (do whats in the brackets first))

(0.00037698 * RPM * r) / (R1 * R2) = km/hr

or to calc RPM when you know speed this way

(km/hr * R1 * R2) / (0.00037698 * r) = RPM

Example:

So in theory (still doesn't measure quite right though)

I think 5th is .63:1? and my diff is 3.27:1 at 100km/hr with 298mm radius


(100 * .63 * 3.27) / (0.00037698 * 298)

206.01 / .11234004

= 1833.81 RPM


Compare with the 322 calc'd measurement of the wheel is 1697.13, so 136rpm of difference. Not much but it makes it more accurate this way...

Hope it helps