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Engine RPM, Wheel RPM & Ideal Road Speed

Relate engine and wheel rotational speed through an overall drive ratio, then estimate road speed from tire diameter.

  • Formula & worked example
  • Private in your browser
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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Private by default

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

With an overall 4:1 drive ratio, 100 wheel rpm corresponds to 400 engine rpm. A 25-inch tire gives an ideal road speed of about 11.97 km/h at that wheel speed.

Engine rpm = wheel rpm × overall ratio; speed in mph = wheel rpm × tire diameter in inches × π × 60 ÷ 63,360

Worked example

Enter these known values and leave the other values blank.

Wheel rotational speed
100 rpm
Overall engine-to-wheel ratio
4
Effective tire diameter
25 in
Engine rotational speed
400 rpm
Ideal road speed
11.97 km/h

Assumptions and limitations

  • The overall ratio is engine revolutions per wheel revolution, including the selected gear and final drive. A 4:1 ratio is entered as 4.
  • The wheel rolls without slipping and has circumference π times the entered positive diameter. Tire deflection, load, wear and drivetrain slip can change actual road speed.
  • RPM means revolutions per minute; hertz means revolutions per second here. One revolution is exactly 2π radians. Zero rotational speed is permitted; signed speeds represent the algebraic direction convention.
  • This model evaluates an entered ratio and rolling diameter. It does not provide vehicle-specific calibration, drivetrain suitability or operating limits.

Common questions

Which ratio should I enter?

Use the overall engine-to-wheel ratio for the selected gear. Multiply the individual reductions that lie between the engine and wheel; do not enter just one stage when other stages also change speed.

Can I calculate wheel speed from engine speed?

Yes. Enter the engine rotational speed and overall ratio. At 400 rpm with a ratio of 4, the wheel rotates at 100 rpm.

References

Bookify uses the exact relationship one revolution = 2π radians instead of the captured rounded angular-speed factor.

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