⚙️ Gear Ratio & RPM Calculator
Enter your gear ratio, final drive (axle) ratio and tire size to find your road speed at a given RPM — or the cruising RPM at a given speed.
Total drive ratio
3.55:1
Road speed
62.8 mph
101 km/h
Total ratio = gear × final drive. Wheel RPM = engine RPM ÷ total ratio; speed = wheel RPM × tire circumference. A numerically higher final drive gives quicker acceleration but higher cruising RPM. 🔒 In your browser.
How the gear ratio & rpm calculator works
The wheels turn at the engine speed divided by the total drive ratio (transmission gear × final drive). Multiply the wheel RPM by the tire’s circumference and you get the distance covered per minute, which converts to road speed. The tool works in both directions — RPM to speed, or speed to the RPM you’d be turning.
A numerically higher final-drive ratio (say 4.10 vs 3.55) gives quicker acceleration but raises cruising RPM and can hurt fuel economy. Larger tires effectively lower the ratio, and smaller tires raise it — which is why tire size is part of the calculation.
Frequently asked questions
How do I calculate RPM from speed?
RPM = (road speed ÷ tire circumference) × total drive ratio, where the total ratio is the transmission gear times the final drive. The tool does the unit conversions and gives the engine RPM for any speed.
What is the final drive ratio?
The ratio of the differential (axle) — how many times the driveshaft turns per wheel revolution. A 3.55 final drive means 3.55 driveshaft turns per wheel turn. Combined with the transmission gear, it sets the total drive ratio.
How does tire size change my RPM?
Larger tires cover more ground per revolution, so for the same speed the wheels — and engine — turn more slowly, effectively lowering your gearing. Smaller tires raise the RPM. That’s why the calculator needs the tire size.
Does a higher gear ratio mean faster acceleration?
A numerically higher final-drive ratio (e.g. 4.10) multiplies torque more, giving quicker acceleration, but the engine spins faster at any given speed — worse for highway RPM and economy. It’s a trade-off.
What RPM should I cruise at on the highway?
It depends on the car, but many modern vehicles cruise around 2,000–2,500 RPM at highway speed in top gear. Enter your top-gear ratio, final drive and tire size to see yours.