Axle ratio change calculator

Why the axle ratio and tire size belong together

Two numbers set how fast your engine turns at a given speed: the combined gear ratio of the transmission and axle, and the height of the tire. Change either and the cruise RPM changes. The RPM to MPH calculator tells you one setup's speed or RPM. This page asks the question you have when you are about to spend money: what happens to my RPM if I change the axle ratio, the tires, or both?

The method: gearing is a product, and tire size is part of it

Engine RPM = (wheel RPM) × (transmission ratio) × (axle ratio). Wheel RPM is the speed divided by the tire's circumference. Putting it together gives RPM = mph × gear × axle × 336.135 ÷ D, where D is the tire diameter in inches. Everything except speed is a ratio, so a change in any one factor changes RPM by the same fraction. A 10 percent taller tire is a 10 percent taller gear; a 10 percent numerically higher axle ratio is a 10 percent shorter gear. This calculator compares the old and new setups at one speed and shows the table for other speeds.

Worked examples

With a 0.75 overdrive, a 3.73 axle and 265/70R17 tires (31.6 inches), the engine turns 2,083 RPM at 70 mph. Fitting 285/70R17 tires (32.7 inches) lowers it to 2,012. Changing only the axle to 4.10 raises it to 2,289, up 9.9 percent. Doing both together gives 2,212 RPM, 6.2 percent above where you started. This is the usual reason people regear after a tire upgrade: the larger tire alone drops RPM so far that the engine lugs when climbing or towing, and a numerically higher axle puts the cruise RPM back near the stock figure.

Common mistakes

Using the rim diameter instead of the overall tire diameter is the most frequent one, because it ignores the sidewall; type the full size, such as 285/70R17, and the calculator works out the tire height. Mixing up the direction of the ratio is the second: 4.10 is a higher number than 3.73 and means the driveshaft turns more times per wheel turn. Third, entering a 1.00 transmission ratio when you cruise in overdrive produces an RPM that is a third too high. Finally, the diameter of a loaded tire is smaller than a new unloaded one, and it shrinks further as tread wears, so the real RPM at speed is a little above the calculated figure.

Limits of this method

The formula assumes no slip, which an automatic transmission's torque converter breaks until the lockup clutch engages: below that speed the engine can turn faster than the gearing says. It also ignores the fact that the speedometer, the transmission shift points and any speed-sensitive controls were calibrated for the old gearing. After a change, have the speedometer recalibrated and check the shift behavior. The speedometer error calculator on this site shows the size of the error. Gearing also changes the load on the drivetrain: a numerically higher ratio multiplies torque at the wheels, which the axle, driveshaft and transmission have to carry.

How do I calculate engine RPM after changing the axle ratio?

RPM = mph × gear ratio × axle ratio × 336.135 ÷ tire diameter in inches. Enter your speed, the ratio of the gear you cruise in, the old and new axle ratios and your tire, and the calculator shows both RPMs and the percentage change. Because the axle ratio multiplies straight through, going from a 3.73 to a 4.10 axle raises RPM by 4.10 ÷ 3.73, about 9.9 percent, at any speed and in any gear.

Where does the number 336 come from?

A mile has 63,360 inches and an hour has 60 minutes. A wheel that turns once rolls π times its diameter, so 1 mph needs 63,360 ÷ (60 × π) = 336.135 inches of diameter times wheel revolutions per minute. People round it to 336. It is geometry, not a measured constant, and it assumes the tire does not slip and that the diameter you enter is the loaded rolling diameter.

How much do bigger tires change my effective gearing?

The overall gearing is the axle ratio divided by the tire diameter, so a taller tire acts like a lower (numerically smaller) axle ratio. Going from a 265/70R17 (31.6 inches) to a 285/70R17 (32.7 inches) at 70 mph in a 0.75 overdrive with a 3.73 axle drops RPM from 2,083 to 2,012, 3.4 percent lower. The same tires would need a 3.86 axle to bring the old RPM back.

What axle ratio do I need for a given cruise RPM?

Use the second mode: axle = RPM × tire diameter ÷ (mph × gear × 336.135). For 2,000 RPM at 70 mph in a 0.75 gear on a 285/70R17 it gives 3.71, so a 3.73 axle would turn about 2,012 RPM and a 4.10 about 2,212. Axle sets come in fixed ratios, so choose the nearest one and run the first mode to check the real RPM.

Will a numerically higher axle ratio hurt my fuel economy?

It raises cruise RPM, and for most engines more RPM at the same speed means more fuel burned, but the effect depends on the engine's efficiency at that point, which this calculator does not model. Heavy loads or towing can reverse it, because the engine works less hard near its torque peak. We give you the RPM; the fuel economy has to be measured.

Does it matter which gear I enter?

Yes. The transmission ratio is part of the product. Enter 1.000 for a direct-drive gear and the overdrive ratio (below 1.000, for example 0.75) for the gear you cruise in. The gear ratios are on the transmission's data plate or in the service manual. The swap changes RPM by the same percentage in every gear, but the starting RPM is gear-dependent.

Sources: the speed formula is geometry (63,360 inches per mile ÷ 60 minutes ÷ π), the same one used by the RPM to MPH calculator; tire height comes from the size code as in the tire size calculator. The axle ratio that restores the old RPM, and the ratio for a target RPM, are our derivations from that formula. Related: speedometer error calculator, gear ratio calculator, horsepower calculator. How we check our calculators.