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RPM Calculator

Computes a gear pair or train ratio from tooth counts with output speed and torque, and the vehicle overall ratio, tyre diameter from the size code, road speed at an engine speed and engine speed at a road speed using exact NIST unit constants.

Engine speed at the road speed
2,779 rpm
Gear Ratio Calculator
Overall ratio (gear x final drive)
2.8660
Tyre sidewall height
92.25 mm
Tyre diameter (nominal, unloaded)
616.3 mm
Tyre diameter (nominal, unloaded)
24.264 in
Tyre circumference
1,936.2 mm
Revolutions per mile used
831.2 rev/mile
Road speed at the engine speed
75.56 mph
Road speed at the engine speed
121.60 km/h
Same speed by the approximate "336" shortcut
75.59 mph
Wheel speed at the engine speed
1,046.76 rpm
Worked out from Carnegie Mellon University, Introduction to Mechanisms, Chapter 7 Gears (velocity ratio is the inverse tooth ratio; train ratio = product of driven / product of driver teeth) and 7 other sources.
The tyre diameter is the nominal, unloaded code dimension. A loaded tyre rolls on a smaller radius and slips slightly, so real revolutions per mile run a few percent higher (the pack's 205/45R17 example: 831 geometric vs 857 published, about 3 %). Wear, pressure and speedometer error are not modelled.

Your figures

Updates as you type
e.g. 2020 MX-5 6MT: 1st 5.087 ... 6th 1.000. Overdrive ratios below 1 are fine.
Multiply in a transfer case or hub reduction as well if fitted.
Metric (205/45R17, P225/70R16 91S, LT265/70R17) or flotation (33x12.50R17). Gives the nominal unloaded diameter.
Leave at 0 to use the geometric diameter. A published figure accounts for the loaded rolling radius and is usually a few percent higher than geometry.

How this is worked out

6 steps
  1. Overall ratio = gear 1 x final drive 2.866 = 2.866
  2. Tyre 205/45R17: sidewall = 205 x 45/100 = 92.25 mm; diameter = 17 x 25.4 + 2 x 92.25 = 616.3 mm = 24.2638 in
  3. Road speed = 3000 rpm x 24.2638 in x pi x 60 / (2.866 x 63,360 in/mile) = 75.56 mph; in metric 3000 x 616.3 mm x pi x 60 / (2.866 x 1,000,000) = 121.601 km/h
  4. Approximate "336" shortcut: 3000 x 24.2638 / (2.866 x 336) = 75.59 mph (63,360 / 60 pi is really 336.1352, so the shortcut reads about 0.04 % high)
  5. Engine speed at 70 mph = 70 mph x 2.866 x 63,360 / (24.2638 x pi x 60) = 2779.3 rpm
  6. Wheel speed = 3000 / 2.866 = 1046.76 rpm

Its 5 worked examples are re-run on every release, last on 15 September 2026. What changed

Sources and references

8 sources
Carnegie Mellon University, Introduction to Mechanisms, Chapter 7 Gears (velocity ratio is the inverse tooth ratio; train ratio = product of driven / product of driver teeth)
Mustansiriyah University, Mechanical Engineering lecture 8: Gears (efficiency = output power / input power; worked example 20 kW, ratio 5, 70 %)
CED Engineering course M06-031, Basic Fundamentals of Gear Drives (20-tooth pinion to 60-tooth gear at 1725 rpm; indicative efficiency ranges)
NIST SP 811 Appendix B.8, conversion factors (inch = 2.54 E-02 m exact; mile = 1.609344 E+03 m exact; horsepower (550 ft lbf/s))
Bridgestone, How to Read Tire Size (width in mm, aspect ratio as a percentage, rim diameter in inches)
Tire Rack, How Do I Calculate Tire Dimensions? (overall diameter = 2 x sidewall + rim; nominal versus actual caveat) and What Is the Revolutions Per Mile of a Tire?
Tire Rack, Racing and Rally Motorsport Tire Size Formats (flotation format 35x12.50R17: diameter x width R rim, inches)
Mazda USA, 2020 MX-5 Miata Features and Specs (6MT ratios, final drive 2.866, P205/45R17)
2020

What this calculator does

Computes a gear pair or train ratio from tooth counts with output speed and torque, and the vehicle overall ratio, tyre diameter from the size code, road speed at an engine speed and engine speed at a road speed using exact NIST unit constants.

How to use RPM Calculator

Enter Transmission ratio in the selected gear, Final drive (axle or differential) ratio, Tyre size code, Manufacturer revolutions per mile (optional), Engine speed for the road-speed result, Road speed for the engine-rpm result, Road speed unit and Headline result. The calculator checks the values, applies the stated method, and updates the result without saving your figures.

What the result means

The result includes Gear ratio (driven / driver), Ratio as n:1, Output speed, Input torque (selected unit), Output torque (selected unit), Torque unit, Output torque, Input power, Output power, Overall ratio (gear x final drive), Tyre sidewall height, Tyre diameter (nominal, unloaded), Tyre diameter (nominal, unloaded), Tyre circumference, Revolutions per mile used, Road speed at the engine speed, Road speed at the engine speed, Same speed by the approximate "336" shortcut, Wheel speed at the engine speed and Engine speed at the road speed. Review the visible working and any warnings before using the answer for a decision.

Method and checks

The method cites Carnegie Mellon University, Introduction to Mechanisms, Chapter 7 Gears (velocity ratio is the inverse tooth ratio; train ratio = product of driven / product of driver teeth); Mustansiriyah University, Mechanical Engineering lecture 8: Gears (efficiency = output power / input power; worked example 20 kW, ratio 5, 70 %); CED Engineering course M06-031, Basic Fundamentals of Gear Drives (20-tooth pinion to 60-tooth gear at 1725 rpm; indicative efficiency ranges), among others. Its 5 built-in worked examples are checked on every full test run. The initial answer and working are rendered on the server, so they remain readable without JavaScript.

Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.

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Copy this code into any page. The calculator appears with the figures currently entered, sizes itself to fit, and a small credit line links back here. Free, no account, no ads inside it, no tracking of your readers. Set your own figures and fixed fields; Personal removes the credit line, sets your colour and shows you which sites use it.

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