Axle ratio from tooth counts
- Input
- Ring gear = 41 teeth, Pinion = 10 teeth
- Working
- Ratio = 41 ÷ 10 = 4.10, written 4.10:1.
- Result
- Final-drive ratio = 4.10:1
Gear Ratio Calculator
Calculate axle gear ratios from ring and pinion teeth, then engine RPM at any road speed — or road speed at any RPM — from total ratio and tyre size. Runs entirely in your browser — your numbers never leave this device.
Final-drive (axle) ratio
3.73 : 1
Total ratio in this gear: 3.17 : 1 (final drive × gearbox ratio)
- Total ratio
- 3.17 : 1
- RPM at 60 mph
- 2457.55
- Speed at 2500 RPM
- 61.04 mph
| Road speed | Engine RPM |
|---|---|
| 10 mph | 409.59 |
| 20 mph | 819.18 |
| 30 mph | 1228.77 |
| 40 mph | 1638.37 |
| 50 mph | 2047.96 |
| 60 mph | 2457.55 |
| 70 mph | 2867.14 |
| 80 mph | 3276.73 |
Need the tyre diameter in inches? The Tyre Size Calculator works it out from the sidewall code — a 205/55R16, for example, is 24.88 inches overall. Figures assume no clutch or torque-converter slip and no tyre growth at speed.
How it works
Axle ratio = ring gear teeth ÷ pinion teeth | Total ratio = axle ratio × gearbox ratio | RPM = speed (mph) × total ratio × 336.135 ÷ tyre diameter (in), where 336.135 = 63,360 ÷ (60π) | Speed = RPM × tyre diameter ÷ (total ratio × 336.135)
| Road speed (mph) | Engine RPM |
|---|---|
| 20 | 819 |
| 30 | 1,229 |
| 40 | 1,638 |
| 50 | 2,048 |
| 60 | 2,458 |
| 70 | 2,867 |
| 80 | 3,277 |
A final-drive ratio is a tooth count before it is anything else: divide the ring gear's teeth by the pinion's and 41 ÷ 10 gives the familiar 4.10:1, meaning the pinion — and with it the propshaft — turns 4.10 times for every turn of the wheels. The gearbox multiplies on top of that: the total ratio the engine actually works against is the axle ratio times the selected gear's ratio, so a 3.73 axle behind a 0.85 overdrive gear is a 3.17 total, and it is the total, never the axle alone, that belongs in any RPM calculation.
Engine RPM at a road speed follows from the tyre. A wheel's revolutions per mile are fixed by its circumference — 63,360 inches in a mile divided by π times the diameter in inches — and multiplying revs per mile by the total ratio and the speed, then dividing by 60, converts wheel speed into crankshaft RPM. The constants collapse into one number, 63,360 ÷ (60π) = 336.135, giving RPM = speed × total ratio × 336.135 ÷ tyre diameter. The formula inverts exactly: speed = RPM × diameter ÷ (total ratio × 336.135), so a question asked either way — how fast at 3,000 RPM, or how many RPM at 60 — is the same arithmetic read forwards or backwards, and this page answers both.
Two inputs deserve care. The tyre diameter is the overall diameter in inches, which the sidewall code determines — a 205/55R16 works out to 24.88 inches on the Tyre Size Calculator — and fitting a taller tyre lowers RPM at every speed exactly as a longer gear would. And the figures assume a locked driveline: an automatic's torque converter slipping at low speed and light throttle adds RPM the formula cannot see, which is why calculated and observed figures agree best at a steady cruise in a locked gear.
Worked examples
RPM at 60 mph
- Input
- Speed = 60 mph, Total ratio = 3.73, Tyre diameter = 26 in
- Working
- RPM = 60 × 3.73 × 336.135 ÷ 26 = 2893.35, so the engine sits at about 2,893 RPM at a steady 60 mph in that gear.
- Result
- Engine speed ≈ 2,893 RPM at 60 mph
Road speed at 3,000 RPM
- Input
- RPM = 3000, Total ratio = 2.80, Tyre diameter = 27 in
- Working
- Speed = 3000 × 27 ÷ (2.80 × 336.135) = 81,000 ÷ 941.18 = 86.06 mph.
- Result
- Road speed ≈ 86.06 mph at 3,000 RPM
Frequently asked questions
What does a higher (numerically larger) gear ratio do?
It multiplies torque at the wheels and raises engine RPM at every road speed: at a total ratio of 3.73 on a 26-inch tyre, 60 mph needs about 2,893 RPM, while the 3.17 total in this page's table needs only 2,458 RPM at the same speed. Shorter gearing pulls harder and cruises louder; taller gearing does the reverse.
Why does tyre size appear in an RPM calculation?
Because the wheel is the last gear in the chain. A taller tyre covers more ground per revolution, so it needs fewer revolutions per mile and the engine turns slower at the same speed — the same effect as fitting a taller axle ratio. Enter the overall diameter in inches; the Tyre Size Calculator derives it from the sidewall code.
Where does the 336.135 constant come from?
From the unit conversions, exactly: a mile is 63,360 inches, a wheel's circumference is π × its diameter, and an hour is 60 minutes. Revs per mile (63,360 ÷ πd) times mph gives revs per hour; dividing by 60 gives RPM. Grouping the constants, 63,360 ÷ (60π) = 336.135, and the formula RPM = speed × total ratio × 336.135 ÷ diameter is that chain in one line.
Why is my car's real RPM higher than the calculation at low speed?
Usually torque-converter slip in an automatic: until the converter locks, the engine turns faster than the locked-driveline formula predicts. Clutch slip, a tyre that is under-inflated or worn (a smaller effective diameter), and tyre growth at speed also move the real figure slightly. At a steady cruise in a locked gear the agreement is close.
Is the axle ratio the same as the total ratio?
No, and confusing them is the classic error. The axle ratio (ring ÷ pinion, such as 41 ÷ 11 = 3.73) is only the final drive. The total ratio multiplies it by the gearbox ratio of the gear you are in — 3.73 × 0.85 = 3.17 in an overdrive gear, but 3.73 × 1.00 = 3.73 in a direct gear. RPM follows the total.
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