Vehicle speed calculator

Use our vehicle speed calculator to calculate the top speed for each gear given the gear ratio, engine speed (RPM) and tyre size.

Input values

Wheel & tyre set up

Engine information

Gear ratios


Tyre Circumference: mm
GearSpeed (MPH)Speed (KMH)

Understanding the different parameters

Wheel & tyre set up

This is where you enter the size of your wheels and tyres. The wheel size is in inches, tyre width in millimetres and the tyre height ratio as a percentage of the width - all of these values can usually be found on the sidewall of your tyre.

These values are used to calculate your overall tyre's circumference, which is then used in the speed calculations.

You can learn more about wheel and tyre fitment as well as visualise different offset options by using our wheel offset calculator.

Engine information

Enter the engine's maximum RPM here, or alternatively play around with this number to calculate the speed at different RPMs.

Gear ratios

Here you can enter your final drive ratio (also known as the differential ratio). As well as the ratios for as many gears as you like. Simply click the "Add Gear" button to add another gear to the calculation, or the red "x" box next to each input to remove a gear.

How to calculate the speed from the tyre size, engine speed and gear ratios

The first step in the process is to calculate the tyre's circumference with this formula.

Tyre circumference =
π × (Wheel size in inches × 25.4 + ((Tyre width in millimetres × Tyre height ratio as a percentage) / 100) × 2)

We then use the tyre circumference value to calculate the speed using this formula.

Speed (KMH) = (((Engine speed as RPM / Gear ratio / Final drive ratio) × Tyre circumference) / 1000000) × 60 Speed

Speed (MPH) = ((Speed KMH) × 0.62 × 100) / 100