Axle Ratio 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.
Road speed at the engine speed
75.56 mph
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
121.60 km/h
Same speed by the approximate "336" shortcut
75.59 mph
Wheel speed at the engine speed
1,046.76 rpm
Engine speed at the road speed
2,779 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.