The formula
speed (mph) = RPM ÷ (primary × gear × rear/front) × tire diameter (in) × π × 60 ÷ 63,360
63,360 is the number of inches in a mile. For km/h, multiply mph by 1.609.
Where to find each number
- Primary ratio – the reduction between crankshaft and gearbox, listed in the service manual’s specifications (often between 3 and 4 on small four-strokes).
- Gear ratio – each gear’s ratio is also in the manual. Use top gear for top speed. On a direct-drive go-kart with a centrifugal clutch, leave primary and gear at 1.
- Tire diameter – the first number of an ATV or kart tire size (23 in 23x7-10). For motorcycle tires like 80/100-12, calculate it or measure the inflated tire.
Why real speed is lower
The result assumes the engine reaches that RPM in that gear and the tire doesn’t slip. Small engines with tall gearing often can’t pull redline in top gear, and soft tires grow or squash at speed – so treat the number as the ceiling.