RPM to Speed Calculator
Vehicle speed from engine RPM.
Enter your values
Enter the engine rpm you are working with.
Enter the gear ratio you are working with.
Enter the final drive ratio you are working with.
Enter the tyre diameter in in.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Speed
- 43.2 mph
- Speed in km/h
- 69.6
- Wheel RPM
- 570
- Tyre circumference
- 80.1 in
Quick answer
Road speed comes from wheel RPM after gearing, multiplied by tyre circumference.
Speed = RPM ÷ (gear × diff) × circumference
At a glance
| What it does | Vehicle speed from engine RPM. |
|---|---|
| Category | Engineering |
| Inputs needed | Engine RPM, Gear ratio, Final drive ratio, Tyre diameter |
| Main output | Speed |
| Formula | Speed = RPM ÷ (gear × diff) × circumference |
| Cost | Free — no sign-up, no download |
How to use the RPM to Speed Calculator
- 1Enter engine RPM and the gear ratio in use.
- 2Enter the final drive ratio.
- 3Enter tyre diameter in inches.
Inputs explained
- Engine RPM
- Enter the engine rpm you are working with.
- Gear ratio
- Enter the gear ratio you are working with.
- Final drive ratio
- Enter the final drive ratio you are working with.
- Tyre diameter(in)
- Enter the tyre diameter in in.
Worked example
Using the values the calculator loads with:
Inputs
- Engine RPM3000
- Gear ratio1.35
- Final drive ratio3.9
- Tyre diameter25.5 in
Results
- Speed43.2 mph
- Speed in km/h69.6
- Wheel RPM570
- Tyre circumference80.1 in
Frequently asked questions
Where do I find tyre diameter?
Calculate it from the sidewall code or measure it loaded.
Why is my speedo different?
Speedometers usually read 2–5% optimistic by design.
What do I need to enter into the RPM to Speed Calculator?
Just 4 values: engine rpm, gear ratio, final drive ratio, tyre diameter. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the RPM to Speed Calculator work out the answer?
Road speed comes from wheel RPM after gearing, multiplied by tyre circumference. It applies the formula Speed = RPM ÷ (gear × diff) × circumference and shows the working so you can check each step by hand.
Is the RPM to Speed Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.