Speedometer Correction Calculator

What this number means

The speedometer counts revolutions of something — a transmission output shaft or the wheels — and assumes factory tires and gears. Bigger tires cover more ground per revolution, so you go faster than it says. Deeper gears spin an output-shaft sensor faster, so it says faster than you go. The two can cancel, stack, or flip — the factor table settles it.

What to change next

A few percent is trivia; past ~5% you want a programmer (Tazer, ProCal, FORScan, Hypertech depending on platform) — not for the needle's sake but because the transmission's shift schedule and the odometer run on the same signal. The Decision System includes per-platform recalibration guides; this page gives you the factor to enter.

Worked example

35s replacing 32.1-inch tires with no gear change: factor 1.090 — an indicated 65 is a true 70.9, and the odometer under-records by 9%. Add 4.56 gears replacing 3.45s on a transmission-sensor Jeep and the gear term swings it the other way: factor 0.824, indicated 65 is a true 53.6. Direction matters — run your real pair above.