RPM Calculator

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.
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.

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

Results are general information, not financial, tax, legal, medical or engineering advice.