Axle Ratio Calculator

The axle tag is missing, painted over, rusted through, or you just don't trust it after a used-axle swap. Two ways to find the real ratio: read it off the tach at a steady speed, or turn the wheel by hand and count driveshaft revolutions — no electronics required for the second one.

Method 1 — from an RPM/speed reading

Drive at a steady speed in a 1:1 (direct-drive) gear — commonly top gear on a manual, or check your transmission's gear ratio chart if you're not sure it's 1:1 — and note the tachometer reading.

Method 2 — driveshaft-turns (no electronics)

Open differential only. Transmission in neutral, parking brake off, both drive wheels off the ground. Mark the driveshaft and one tire. Turn that tire exactly 2 full revolutions while counting driveshaft revolutions — the differential's averaging with the other free-spinning wheel makes the ratio come out directly. A locked or limited-slip axle needs both wheels turned together and won't read correctly this way.

Both methods estimate the ratio from a physical measurement, so a reading a little off a catalog value (say 3.71 instead of 3.73) almost always means you're reading a real 3.73 gear set with normal measurement error, not a genuinely uncommon ratio — that's why each result below is matched to the closest ratio actually sold.

Why the RPM/speed method needs a 1:1 gear

Total gearing at any moment is the transmission ratio multiplied by the axle ratio. In a 1:1 gear that multiplication does nothing, so the reading is the axle ratio directly. In any other gear you need that gear's exact ratio to divide back out — this calculator accepts it as an input for exactly that reason, defaulting to 1.00 for the common case.

Already know your ratio? See what a tire change does to it →
Regearing for bigger tires? Find the ratio to buy →