Drivetrain / Speed
Gear Ratio Speed Calculator: RPM, Tyres and Final Drive
Enter tyre size or direct diameter, final drive and up to ten gear ratios. Compare speed at an engine RPM, RPM at a road speed and speed-versus-RPM curves.
Gear Ratio Speed Calculator: RPM, Tyres and Final Drive: Combines tyre circumference with gear and final-drive reductions. It calculates road speed for each gear at the chosen RPM and engine RPM at the chosen road speed. It assumes no tyre or drivetrain slip; available power and drag can limit actual top speed. Runs 100% locally in your browser with zero server file uploads.
- Category
- Everyday tools
- Runs
- In your browser
- Cost
- Free · no sign-up
- Availability
- Ready to use
Runs entirely in your browser
Nominal tyre diameter: 634.3 mm
| Gear | Ratio | km/h | mph | RPM |
|---|---|---|---|---|
| 1 | 3.6 | 24.3 | 15.1 | 12345 |
| 2 | 2.1 | 41.7 | 25.9 | 7201 |
| 3 | 1.4 | 62.5 | 38.8 | 4801 |
| 4 | 1 | 87.5 | 54.4 | 3429 |
| 5 | 0.8 | 109.4 | 68.0 | 2743 |
| 6 | 0.65 | 134.6 | 83.6 | 2229 |
Speed (km/h) = RPM × π × diameter (mm) × 60 ÷ (gear ratio × final drive × 1000000). Reverse the equation for RPM. Ratios are input turns per output turn.
Kinematic estimate with no tyre slip or drivetrain slip. Nominal diameter differs from loaded rolling diameter. These speeds are not predictions of achievable top speed.
Rotational relationship
Speed in km/h = RPM × π × tyre diameter in mm × 60 / (gear ratio × final drive × 1000000). RPM is the inverse. For a tyre with exactly one metre circumference, 1000 engine RPM, a 2:1 gear and 4:1 final drive give 7.5 km/h.
Automotive reference
John Lawlor and Bill Hancock, Auto Math Handbook, revised edition, HP Books, 2011, chapter MPH, RPM, gears and tires. Bibliographic record: https://libcatalog.neit.edu/cgi-bin/koha/opac-ISBDdetail.pl?biblionumber=71599 . The calculation uses nominal geometry, not a measured loaded rolling radius.
How to use it
- Enter a tyre size such as 225/45R17 or a direct diameter in millimetres.
- Enter final drive and up to ten comma-separated gear ratios.
- Set engine RPM and road speed to read both directions and the chart.
Privacy & limitations
Gearing and tyre inputs are calculated locally in your browser.
Related tools
Frequently asked questions
Which way round are the ratios?
Enter input turns per output turn. For example, a 3:1 gear and 4:1 final drive turn the wheel once per twelve engine revolutions.
Can I use a measured diameter?
Yes. Select direct diameter and enter millimetres. A nominal sidewall size does not fully describe loaded rolling circumference, tyre wear or growth at speed.
Is this a top-speed prediction?
No. The chart is a geometric relationship between RPM and road speed. It does not account for power, drag, traction, clutch slip or torque-converter slip.
Free tool · runs in your browser · no account required