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Tire Size Math Tire size and gear ratio math.

Gear ratio calculator

Work out an axle gear ratio from the ring and pinion tooth counts, and the engine RPM it gives at highway speeds with your tire size.

Example: 41 ÷ 10 teeth

Axle ratio: 4.10:1

Engine RPM on 265/70R17 (31.61 in), 1:1 gear

Road speedEngine RPM
30 mph1,308
45 mph1,961
60 mph2,615
75 mph3,269

The 336 constant assumes the transmission is in a 1:1 gear. Most modern transmissions have an overdrive top gear below 1:1 (often 0.6 to 0.85); enter it as the transmission ratio to include it.

Check the vehicle's door placard and owner's manual, and a tire professional, before changing sizes.

Add an overdrive gear or a new tire size

How it's calculated

336 comes from 63,360 inches per mile ÷ (60 minutes × π) = 336.14, rounded the way most gear charts do. It is about 0.04% low, under one RPM at highway speed.

The tooth counts are usually stamped on the ring gear or the pinion. Many axles also have a tag on a cover bolt with the ratio.

Frequently asked questions

What does a 4.10 gear ratio mean?

The driveshaft turns 4.10 times for each turn of the wheels. A 41-tooth ring gear driven by a 10-tooth pinion gives 4.10:1.

Is a higher or lower number better for towing?

A numerically higher ratio (4.10 vs 3.73) multiplies torque more and turns the engine faster at a given speed. A lower one cruises at lower RPM. The right balance depends on the vehicle, the load and the tire size.

Why is the RPM table for a 1:1 gear?

The 336 formula assumes the transmission is in a gear where the output shaft turns at engine speed. Most top gears are overdrive (below 1:1), so cruising RPM is lower. The effective gear ratio calculator takes the transmission ratio.

How do I find the ratio without counting teeth?

Lift a rear wheel, mark the driveshaft and tire, turn the tire once by hand and count the driveshaft turns (open differentials need both rear wheels off the ground and turned together).

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