How to Convert Engine RPM to Road Speed

This guide explains how to convert engine RPM into road speed 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 wheel diameter, you can calculate rear wheel speed and convert it into miles per hour. The article also includes a spreadsheet tool that performs the calculation automatically.

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

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 sprocket.

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 Engine RPM to Rear Wheel RPM

Multiply the number of teeth on the clutch sprocket and rear sprocket together. Do the same with the number of teeth on the engine and gearbox sprockets. Divide as in the equation shown here.

This gives the ratio between the engine  speed and the rear wheel speed. Multiply this answer by the engine speed to get the rear wheel 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 divide your answer by the ratio. For example if your gearbox ratio is 2:1 divide your answer from the above by 2 to get your engine RPM.

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Convert Rear Wheel RPM to Inches Per Minute

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

Circumference = Diameter x π

(π=3.142)

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Convert Inches Per Minute to Miles Per Hour

Multiply by 60 to get inches per hour, then divide by 63360 to get miles.

(There are 63360 inches in 1 mile)

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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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