How to calculate vehicle speed from engine RPM and vice versa

Suppose your motor vehicles speedometer not working and you need to know your vehicle speed for the given engine speed for a certain gear ratio. How you can do that , let’s see..

Say, Engine rpm= 3000
Gear ratio=1.5
Axle ratio = 4
Tyre size = 12 inch radius

The formula for calculating vehicle speed is:
Car or automobile speed in consistent unit,
V = (engine rpm * wheel tyre perimeter)/(gear ratio * axle ratio)

Or, V = (engine rpm * 3.14 * tyre diameter ) / ( gear ratio * wheel axle ratio)

For the given values,
V  = (3000 rpm* 3.14 * 2 * 12 inch) / ( 1.5 * 4)

Or, V (kmph)= (3000 * 60 rph * 3.14 * 2 * 12 * 25.4 * 0.000001 km)/(1.5 * 4)

Or, V = 57.42 kmph

Now as you know the equation for calculating vehicle speed given the engine rotation speed, you can easily find out the engine rpm of your car if you the vehicle speed from your speedo.

Shibashis Ghosh

Hi, I am Shibashis, a blogger by passion and engineer by profession. I have written most of the articles for mechGuru.com. For more than a decades i am closely associated with the engineering design/manufacturing simulation technologies.
Disclaimer: I work for Altair. mechGuru.com is my personal blog. Although i have tried to put my neutral opinion while writing about different competitor's technologies, still i would like you to read the articles by keeping my background in mind.

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