Quick change gear ratio and RPM to MPH calculator

How to use it

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Top gear and bottom gear

Gear charts name the two change gears in different ways, often top and bottom. Which one sits on the input shaft depends on the rear end. This calculator asks which shaft each gear is on, so check your rear end or its gear chart before you enter them.

Swapping the two gears gives a different ratio. If the input shaft gear is smaller than the pinion shaft gear, the final drive ratio goes up.

Tire size changes speed

At the same RPM and ratio, a taller tire covers more ground per turn, so the car goes faster. Measure the tire you will race at race pressure.

Common questions

Which change gear goes on which shaft?

It depends on the rear end. Look at your rear end or its gear chart. The calculator asks for the gear on the input shaft and the gear on the pinion shaft so the math is right either way.

What do I enter for the transmission?

Use 1 for a direct-drive car or a 1:1 high gear. For another gear, enter that gear's ratio.

Which rear tire size should I use with stagger?

With a locked rear end both rear tires turn at the same RPM, but the bigger right rear covers more ground. For a quick estimate, use the average of the two rear tires.

Where does the number 1056 come from?

There are 63,360 inches in a mile and 60 minutes in an hour. 63,360 divided by 60 is 1056.

How exact is the MPH number?

It assumes no tire slip and no tire growth. Real speed can differ a little, so use it to compare gear choices.