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Road Speed to RPM

Converting road speed to engine speed, or engine speed to road speed, is a fairly simple exercise. All you need to know is the diameter of your rear wheel, the number of teeth on each sprocket and the gearbox ratio (usually 1:1 in top gear). This page offers a breakdown of how to do this calculation yourself. There is also a spreadsheet to download that will work it all out for you. 

Required Information

Simple, measure the outside diameter of the rear wheel, in inches, but make sure you also include the tire. 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.

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Notation

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

Convert Road Speed to Engine Speed

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.

IPM.JPG
Convert miles per hour to inches per minute

Divide by 60 to get miles per minute, then multiply by 63360 to get inches.

 (There are 63360 inches in 1 mile)

rpm.JPG
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)

Engine RPM.JPG
Convert 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.

Convert Engine Speed to Road Speed

Although it is less common, if you have a bike that only has a tachometer but would like some idea of your road speed the same principle as above can be used in reverse to find your road speed from a known engine speed. 

rpm back.JPG
Convert engine rpm to 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.

ipm back.JPG
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)

mph back.JPG
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)

Feeling baffled?

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 do all the hard work. Simply input the information from your bike and get the answers calculated for you. 

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