Gear Ratio from RPM
Calculator
Infer overall drivetrain reduction from measured engine RPM, road speed and tire diameter under steady, no-slip conditions.
Your setup
Your results
CalculatedThe inferred ratio includes every reduction between the measured engine shaft and the road.
Clutch or converter slip and speedometer error distort the result. Not valid during a shift or active converter ratio change.
How the calculation works
Follow the formula and keep the input units consistent at every step.
R = engine RPM × π × D / (60 × speed)- Measure steady engine RPM and road speed at the same instant.
- Convert speed to m/s and diameter to meters.
- Rearrange the wheel-speed equation to solve for overall reduction.
A Worked Example
3,600 rpm, 30 MPH and a 13-inch tire imply an overall reduction of approximately 4.64:1.
What Your Result Means
The inferred ratio includes every reduction between the measured engine shaft and the road.
Assumptions & Limitations
Clutch or converter slip and speedometer error distort the result. Not valid during a shift or active converter ratio change.
Common Mistakes
Do not mix wheel RPM with engine RPM, or MPH with m/s.
Questions about this calculation
Which values should I enter?
Measure steady engine RPM and road speed at the same instant. Do not mix wheel RPM with engine RPM, or MPH with m/s.
How should I interpret the result’s precision?
Displayed results are rounded for readability. Intermediate calculations retain floating-point precision; extra displayed decimals do not improve your input accuracy.
Can I use metric units?
Yes. Select Metric above the calculator to convert your inputs. Results always state their units. Tire-size designations keep their conventional mm and inch units.
What are the main limitations?
Clutch or converter slip and speedometer error distort the result. Not valid during a shift or active converter ratio change.
Method, sources & relevant guides
Angular speed and tangential speed. Gear reductions follow tooth engagement and conservation of rotation count.
OpenStax — University Physics: rotation with constant angular accelerationMethod documentation updated October 18, 2026. Independent professional review has not been claimed. Read the calculation and review policy.