Your engine speed at cruise depends on the gearing between the crankshaft and the wheels. Lower (numerically higher) axle ratios and bigger tyres change it, which affects fuel economy, towing and noise. Typical highway cruise is 1,800–2,800 rpm.
How to use the RPM Calculator
- Vehicle speed
- Tyre diameter
- Transmission gear ratio
- Axle ratio
- Every result field is editable: type into Engine speed and Vehicle speed plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the speed, tyre diameter, the ratio of the gear you are in and the axle (final-drive) ratio. Type an rpm into the result to find the speed.
Tips
- Use your own measured figures – fuel economy, speed and prices vary a lot with driving style, traffic, load and season.
- Pick the unit in each field’s menu (miles or kilometres, mpg or L/100 km); the calculator converts everything for you.
- Fill up to the brim twice and divide the distance by the fuel added to measure real fuel economy.
- For vehicle modifications, treat results as a starting point and confirm with the parts maker or a professional mechanic.
Common mistakes to avoid
- Mixing US and imperial (UK) gallons – a UK gallon is about 20% bigger.
- Using the advertised fuel economy instead of real-world figures.
- Forgetting tolls, parking and wear and tear when comparing driving with public transport.
Inputs explained
| Field | What to enter |
|---|---|
| Vehicle speed | Units: mph, km/h, m/s, knots |
| Tyre diameter | Units: in, cm, mm, ft, m |
| Transmission gear ratio | A number |
| Axle ratio | A number |
What each result means
- Wheel speed = [Vehicle speed]÷(pi×[Tyre diameter])×60
- Engine speed = [Wheel speed]×[Transmission gear ratio]×[Axle ratio]
Example values
How the main result changes when Vehicle speed changes and everything else stays at the example values above:
| Vehicle speed (mph) | Engine speed (rpm) |
|---|---|
| 32.5 | 1,019 |
| 48.75 | 1,528 |
| 65 | 2,037 |
| 81.25 | 2,547 |
| 97.5 | 3,056 |
| 130 | 4,075 |
Formula
Wheel rpm = speed ÷ (π × tyre diameter) × 60Engine rpm = wheel rpm × gear ratio × axle ratioWorked example
Inputs — Vehicle speed: 65 mph; Tyre diameter: 28 in; Transmission gear ratio: 0.7; Axle ratio: 3.73.
Results — Wheel speed = 780 rpm; Engine speed = 2,037 rpm.
Frequently asked questions
How do I calculate engine rpm from speed?
rpm = mph × gear ratio × axle ratio × 336 ÷ tyre diameter (in).
Will bigger tyres lower my rpm?
Yes – a larger diameter covers more ground per turn, lowering rpm at the same speed.
What is a good cruising rpm?
1,800–2,500 rpm for most petrol engines, lower for diesels.
What formula does the RPM Calculator use?
Wheel rpm = speed ÷ (π × tyre diameter) × 60.