Water-Cooled VW Performance Handbook

Calculating MPH in gear

You might think that calculating your speed in any given gear a waste of time — why not just go out on the road and see what happens? However, in racing you often need to know how to set up your transaxle to hit certain targets. For example, if you want to be able to make a 200 mph pass at Bonneville, you must first ensure that your motor has the legs to do it.

RPM
Tire diameter		inches
Final drive		:1
Gear ratio		:1
Maximum speed		MPH

For example, at 5,000 RPM, with a tire 22.76 inches in diameter, with a 3.90 final drive ratio and a 0.68 fifth gear, your vehicle is capable of a top speed of 128 MPH.

The formula for the maximum speed you can reach is:

\[Maximum\ MPH = \frac{RPM \times tire\ diameter \times \pi}{final\ drive\ ratio \times gear\ ratio \times 1056}\]