Final Drive Speed Calculator
The road speed an engine produces in a given gear is pure geometry: it depends on how fast the engine turns, how much the transmission and final drive reduce that speed, and how far the tire rolls per revolution. This calculator takes engine RPM, the selected transmission gear ratio, the final drive (differential) ratio and tire diameter, then returns wheel RPM and theoretical road speed in both miles per hour and kilometres per hour. It assumes no tire slip, which is the standard basis for comparing gearing.
Final drive speed formula
Wheel RPM = engine RPM / (gear ratio * final drive)
Circumference (in) = pi * tire diameter
Speed (mph) = wheel RPM * circumference * 60 / 63,360
Speed (km/h) = mph * 1.609344
The overall reduction is the product of gear and final drive ratios. Wheel revolutions per minute times the tire circumference gives inches per minute; converting with 60 minutes per hour and 63,360 inches per mile yields mph. The mile to kilometre factor 1.609344 is an exact definition.
Final drive context
- Passenger car final drive ratios typically range from about 2.7:1 to 4.5:1.
- Top gear ratios in modern transmissions are often below 1.0 (overdrive) to reduce highway RPM.
- Larger tire diameter raises road speed per engine RPM and effectively lengthens the gearing.
- The calculation assumes no slip, the standard basis for theoretical speed.
- Use the loaded (rolling) tire diameter rather than the unloaded sidewall figure for best accuracy.
Final drive speed: frequently asked questions
How do you calculate speed from RPM and gear ratios?
Wheel RPM equals engine RPM divided by the product of the transmission gear ratio and final drive ratio. Multiply wheel RPM by the tire circumference to get distance per minute, then convert to your speed unit. This calculator does all of that for you from your inputs.
What is the final drive ratio?
The final drive ratio is the gear reduction in the differential between the driveshaft and the axle, typically between about 2.7:1 and 4.5:1 for passenger cars. Combined with the transmission gear ratio, it sets how many engine revolutions produce one wheel revolution.
How is tire diameter used?
Tire diameter sets the rolling circumference (pi times diameter), which is the distance the wheel travels per revolution. A larger diameter covers more ground per turn, raising road speed at a given wheel RPM. Enter the loaded diameter of your tire for best accuracy.
Does this account for tire slip or driveline loss?
No. This is a geometric calculation assuming no slip and rigid gearing, which is the standard way to compute theoretical road speed for a given gear. Real-world speed is marginally lower due to tire deformation and slip, typically a fraction of one percent.
Why convert to mph this way?
One mile is 63,360 inches and one hour is 60 minutes. If tire diameter is in inches, wheel revolutions per minute times pi times diameter gives inches per minute; multiplying by 60 and dividing by 63,360 converts to miles per hour. All constants are exact unit definitions.
Official sources
- U.S. National Institute of Standards and Technology: Unit Conversion Factors.
- U.S. Department of Energy: Drivetrain and Gearing Basics.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.