Free wheel and tyre tools
Gear speed and RPM calculator
Calculate mph and km/h from RPM, gear ratio, final drive and tyre size. Find cruising RPM at a chosen speed and compare up to six forward gears.
How it works
Wheel RPM is engine RPM ÷ gear ratio ÷ final drive. Multiply by nominal tyre circumference and 60, then divide by 1,000,000 for km/h when circumference is in millimetres. Divide km/h by 1.609344 for mph. Reversing the same formula gives engine RPM at a chosen speed. The calculation assumes a fully engaged clutch or locked torque converter, no wheel slip and a 1:1 transfer case. It predicts gearing, not an achievable top speed.
References
A worked example
With 225/45 R17 tyres, a 1.00 gear ratio and a 4.10 final drive, 3,000 rpm gives about 87.48 km/h or 54.36 mph. At 60 mph the same setup needs about 3,311 rpm. These are nominal values; loaded rolling circumference can change the result.
Quick reference
Road speed in mph per 1,000 rpm, by overall ratio
| Tyre | 2.5:1 | 3:1 | 3.5:1 | 4:1 |
|---|---|---|---|---|
| 205/55 R16 | 29.6 | 24.7 | 21.1 | 18.5 |
| 225/45 R17 | 29.7 | 24.8 | 21.2 | 18.6 |
| 225/40 R18 | 29.9 | 24.9 | 21.3 | 18.7 |
| 255/35 R19 | 31 | 25.8 | 22.1 | 19.4 |
| 265/70 R17 | 37.6 | 31.3 | 26.9 | 23.5 |
Overall ratio is gear ratio × final drive. Nominal circumference, no slip.
Explained in our guides
- Plus-sizing wheels: tyre diameter and speedometer changesWork through a 17 to 18 inch wheel change, compare tyre diameters and speedometer effects, and understand why a 3% rule cannot approve fitment.
- How to read a tyre size, load index and sidewall markingsDecode tyre width, aspect ratio, rim diameter, load and speed markings, then calculate sidewall height and diameter using a 225/45 R17 example.
Questions
Where can I find my gear ratios?
Use the transmission and final-drive specifications for your exact car, gearbox and differential. The example ratios in this tool are illustrative and are not a specification for any car. Leave unused gear fields empty.
Does this predict my top speed?
No. It shows road speed corresponding to engine RPM with no slip. Power, drag, gearing, rev limits and tyre ratings can prevent the car reaching that speed. Keep testing within appropriate legal and controlled conditions.
Why does my tachometer show a different RPM?
The tyre's actual rolling circumference differs from nominal geometry. Torque-converter or clutch slip, instrument error and an extra transfer reduction can also change the relationship. Include any transfer ratio in the effective final drive.
Keep checking
Use these results to plan a setup. Confirm fitment, tyre approval and clearances with a qualified professional before buying or fitting parts.