Gear Ratio & RPM
Engine RPM, gear ratio, final drive, and tire diameter into vehicle speed
About this calculator
The Gear Ratio & RPM Calculator turns engine speed into road speed through the drivetrain. From engine RPM, the current gear ratio, the differential (final drive) ratio, and tire diameter in inches, it computes the overall final drive, wheel RPM, tire circumference, and vehicle speed in both mph and km/h. Useful for choosing rear-end gears, comparing transmission ratios, or checking cruise RPM after a tire size change. Non-positive inputs return an error.
How to use the Gear Ratio & RPM calculator
- Enter your values into the input fields.
- Read the result — it updates instantly as you type.
- Adjust the inputs to compare scenarios.
- Copy the page URL to share the exact calculation.
Common examples
- 3000 RPM, 1.00 gear, 3.73 diff, 28 in tires → 67.0 mph (107.8 km/h), wheels at 804 RPM
- 2500 RPM in 0.68 overdrive, 3.42 diff, 26 in tires → 83.2 mph (133.8 km/h)
- 6000 RPM in 1st (2.66), 3.73 diff, 28 in tires → 50.4 mph — final drive 9.92:1
Frequently asked questions
How does the Gear Ratio & RPM calculator work?
Enter your numbers and the calculator applies the standard formula for this everyday task, showing the result and how it was reached. It handles the unit and rounding details so you do not have to.
When would I use the Gear Ratio & RPM calculator?
Whenever this comes up in daily life — planning, shopping, home projects, travel or work — and you want a quick, dependable answer instead of estimating or working it out by hand.
How accurate are the results?
The math runs at full precision internally and only the displayed value is rounded for readability. For real-world use the practical limit is the accuracy of the numbers you enter, not the calculation.
Can I share a specific calculation with someone else?
Yes. Every value you enter updates the URL, so copying the address bar and sharing the link reopens the exact same calculation for whoever clicks it.