How to Convert Road Speed to Engine RPM

This guide explains how to convert road speed into engine RPM using the gearing and rear wheel size of your motorcycle. By using the number of teeth on the engine, clutch, gearbox and rear sprockets together with the rear wheel diameter, you can convert miles per hour into rear wheel speed and calculate the corresponding engine RPM. A downloadable spreadsheet is also included to automate the calculation.

Using Motorcycle Gearing to Calculate Engine RPM

This is very useful when you are tuning a road bike that doesn't have a tachometer. You can get some idea of what revs you are hitting in each gear just by looking at your road speed.

If you want to perform the opposite calculation, see our guide on converting engine RPM to road speed.

Required Information for the Calculation

Simply measure the outside diameter of the rear wheel, in inches, but make sure you also include the tyre. So if you have a 19'' rim, your total diameter will be several inches bigger than this.

Count the number of teeth on each sprocket. That's engine, clutch, gearbox and rear.

Notation and Symbols

C = wheel circumference in inches (including tyre)

CS = number of teeth on clutch sprocket

D = wheel diameter in inches (including tyre)

erpm = engine speed in revs per minute

ES = number of teeth on engine sprocket

GS = number of teeth on gearbox sprocket

ipm = road speed in inches per minute

mph = road speed in miles per hour

R = gearbox ratio (Top gear is usually 1:1)

RS = number of teeth on rear sprocket

wrpm = rear wheel speed in revs per minute

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​Convert Miles Per Hour to Inches Per Minute

Divide by 60 to convert miles per hour to miles per minute, then multiply by 63,360 to convert miles into inches.

(There are 63360 inches in 1 mile)

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Convert Inches Per Minute to Wheel RPM

Divide your speed in inches per minute by the circumference of the rear wheel to find the rear wheel speed in RPM.

Circumference = Diameter x π

(π=3.142)

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Convert Rear Wheel RPM to Engine RPM

Now multiply the number of teeth on the gearbox and engine sprockets together. Do the same with the number of teeth on the rear sprocket and the clutch sprocket. And divide as in the equation shown here.

This gives the ratio between the rear wheel speed and the engine speed. Multiply this answer by the rear wheel speed to get the engine speed in RPM.

This assumes a ratio of 1:1, usual for top gear in the gearbox. If you are using a different gear simply multiply your answer by the ratio. For example if your gearbox ratio is 2:1 multiply your answer from the above by 2 to get your engine RPM.

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Spreadsheet Tool for Automation

If you have read all of the above and are left feeling confused, or would rather be out on your bike than sitting indoors with a pen and paper, we’ve created a spreadsheet to do all the hard work. Simply enter the sprocket sizes, wheel diameter and engine speed from your motorcycle and get the answers calculated for you. 

Road Speed to Engine Speed.xlsx

Download XLSX • 75KB

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