Practical tools for measured work
Gear Ratio, Axle & RPM Calculator
Compare road speed and engine RPM across six transmission gears using tire diameter and axle (final-drive) ratio.
Results assume no tire growth, clutch/converter slip or wheelspin.
How it works
How to use it: enter loaded tire diameter, final drive and each transmission ratio. One input RPM produces road speed in every gear; one road speed produces engine RPM in every gear.
The transmission gear ratio and final-drive ratio are separate reductions. Multiply them to obtain the overall ratio between engine and wheels. A final drive of 4.10 is entered as 4.10, not 4.10 multiplied by the selected gear.
For gearing-limited top speed, enter the engine RPM limit. The highest calculated speed across the entered gears is only a kinematic ceiling. Engine power, aerodynamic drag, rolling resistance, electronic limits, tire ratings and available traction can prevent the vehicle from reaching it.
Formula. Wheel RPM = engine RPM / (gear ratio x final drive). Speed = wheel RPM x tire circumference x 60. The reverse calculation rearranges the same relationship.
Example. A 635 mm tire, 1.00 gear and 4.00 final drive travels about 89.8 km/h at 3000 RPM.
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- Gearing task
- Speed and RPM table
- Loaded tire diameter
- 635 mm
- Final-drive ratio
- 4.1
- Engine RPM for speed results
- 3000 rpm
- Road speed for RPM results
- 100 km/h
- Gear 1 ratio
- 3.5
- Gear 2 ratio
- 2.1
- Gear 3 ratio
- 1.4
- Gear 4 ratio
- 1
- Gear 5 ratio
- 0.8
- Gear 6 ratio
- 0.67
Primary result: Gear 1 speed @ 3000 rpm: 25.0 km/h.
Before using the result
- Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
- Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
- When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.
Limitations
This is drivetrain gearing, not a tire-fitment tool. Use measured loaded/rolling diameter where possible. Automatic-converter slip, clutch slip, tire deformation, growth and speedometer calibration can make real readings differ.
Frequently asked questions
Should I enter advertised tire diameter?
A measured loaded or rolling diameter gives a better result because real tires rarely match the nominal figure exactly.
Where do I find my axle or final-drive ratio?
It is usually stamped on an axle tag or door-jamb code, listed in the build sheet, or available from the axle or transmission manufacturer's catalog for your specific part number.
I only need a quick mph-to-rpm check, not all six gears - is there a simpler tool?
Yes - the Speed to RPM Converter solves the same relationship for one gear ratio at a time, which is faster when you already know the single ratio you care about.
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