Enter a size like 265/70R16 (metric) or 33x12.5R15 (imperial), or just a diameter. Compare two to see speedo error and how it changes your effective gearing.
Engine RPM calculator
Work out cruising revs for a given diff ratio, tyre and speed. Gearbox ratio is your top-gear ratio (1.00 = direct; overdrive boxes are ~0.70–0.85).
Diff ratio finder
Tell us the cruising revs you want at a given speed on a given tyre, and we'll work out the ideal diff ratio — plus the closest common ratio.
