Gears translate engine revolutions into wheel speed. Knowing the speed in each gear at the redline or at cruising rpm helps choose ratios for racing, towing or economy.
How to use the Transmission Speed Calculator
- Engine speed (rpm)
- Transmission gear ratio
- Final drive (differential) ratio
- Tyre diameter
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the engine rpm, the gear ratio, the final drive ratio and the overall tyre diameter.
Inputs explained
| Field | What to enter |
|---|---|
| Engine speed (rpm) | A number |
| Transmission gear ratio | A number |
| Final drive (differential) ratio | A number |
| Tyre diameter | Units: in, mm, cm, ft, m |
What each result means
- Vehicle speed = [Engine speed (rpm)]÷60×pi×[Tyre diameter]÷([Transmission gear ratio]×[Final drive (differential) ratio])
- Wheel speed = [Engine speed (rpm)]÷([Transmission gear ratio]×[Final drive (differential) ratio])
- Engine rpm per 100 km/h = 100÷3.6÷(pi×[Tyre diameter])×60×[Transmission gear ratio]×[Final drive (differential) ratio]
Example values
How the main result changes when Engine speed changes and everything else stays at the example values above:
| Engine speed (rpm) | Vehicle speed (km/h) |
|---|---|
| 1,500 | 50.06 |
| 2,250 | 75.09 |
| 3,000 | 100.1199 |
| 3,750 | 125.1499 |
| 4,500 | 150.1799 |
| 6,000 | 200.2399 |
Formula
v = rpm × π × tyre diameter / (gear ratio × final drive ratio)
Worked example
Inputs — Engine speed (rpm): 3,000; Transmission gear ratio: 1; Final drive (differential) ratio: 3.73; Tyre diameter: 26 in.
Results — Vehicle speed = 100.1199 km/h; Wheel speed = 804.29 rpm; Engine rpm per 100 km/h = 2,996 rpm.
Key terms
- Final drive
- The differential gear reduction.
Frequently asked questions
How do I find tyre diameter from its size?
Use our tyre size calculator, or diameter = rim + 2 × width × aspect ratio.
What does a lower final drive do?
A numerically higher ratio gives more acceleration but lower top speed per rpm.
Does tyre wear change the speed?
Slightly – worn tyres are a little smaller.
Why does the speedometer differ?
Many read a few percent high by design.