Axle Ratio Calculator

The axle tag is missing, painted over, rusted through, or you just don't trust it after a used-axle swap. This axle ratio calculator gives you 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)

Transmission in neutral, parking brake off. Mark the driveshaft and one tire, then turn that tire by hand while counting driveshaft revolutions. Open differential: jack up one drive wheel only and leave the other on the ground — turn the raised tire 2 full revolutions. With one side held still, the ring gear turns only once for those 2 tire turns, so 3¾ driveshaft turns means a 3.73 axle. Limited-slip or locked: raise both drive wheels (they turn together) and turn one tire 1 full revolution — the driveshaft count is the ratio directly. Pick your differential type below; the math changes with it.

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.

Axle ratio questions

How do you calculate axle ratio?

Drive in a 1:1 gear, note engine RPM and road speed, and divide. A reading of 2100 rpm at 65 mph on a 31.6-inch tire works out to 3.04, which is a 3.07 gear set read with normal measurement error. The second method needs no electronics: lift both drive wheels, turn them two full revolutions and count driveshaft turns.

Can I look up my gear ratio by VIN?

Not reliably. The VIN encodes the model, engine and assembly plant, not the axle that was fitted, and an axle swap leaves the VIN unchanged. The axle tag, the door-jamb RPO code or a dealer build sheet do state it. When all three are missing or illegible, measure it with either method above.

Which is better, 3.73 or 4.10 axle ratio?

Neither in general: 4.10 trades cruising rpm for pulling power. On a 31.6-inch tire at 65 mph a 3.73 turns about 2579 rpm and a 4.10 about 2835 rpm, roughly 10% more. Pick 4.10 for towing and larger tires, 3.73 for highway economy.

Does a bigger tire change my axle ratio?

Not the gear set, but it changes what that gear set does. A taller tire behaves like a numerically lower ratio, which is why a tire upgrade often needs regearing to get the stock feel back.

Already know your ratio? See what a tire change does to it →
Regearing for bigger tires? Find the ratio to buy →
Off-road? Work out your crawl ratio →