Vehicle Speed from RPM

This formula converts engine speed into road speed by chaining together two ideas: the gearbox and final drive reduce the engine’s rpm before it reaches the wheels, and the tyres turn wheel rotations into distance travelled. The tyre circumference is c0d, so each wheel revolution moves the vehicle forward by that distance; multiplying by wheel revs per minute and converting through the gear ratios gives road speed. It works best when the tyre diameter is the effective rolling diameter, not just the nominal tyre size printed on the sidewall.

Use this when estimating how fast a vehicle should travel at a given engine rpm in a chosen gear, or when comparing gearing changes such as different differential ratios or tyre sizes. It is also useful for diagnosing speedometer error or checking whether a vehicle’s gearing suits cruising, towing, or acceleration.

• Using tyre diameter in inches without converting it to metres or consistent units
• Forgetting that gear ratio and final drive both reduce wheel speed, so both must be included
• Using the nominal tyre size instead of the actual rolling diameter, which can shift the result noticeably