Gear Ratio Calculator
Enter engine RPM, transmission ratio, final-drive ratio, and tire diameter to get wheel RPM, vehicle speed, overall ratio, and torque multiplication.
Advanced options (full gear set, transfer case, final-drive comparison)
Gear-by-gear comparison
Speed vs. engine RPM (selected gear)
How it works
The overall ratio multiplies every reduction stage between the engine and the wheels:
Overall ratio = transmission ratio × final-drive ratio × transfer-case ratio
Wheel RPM is engine RPM divided by the overall ratio. Vehicle speed converts wheel RPM to distance per minute using the tire's circumference (π × diameter), then to miles per hour. Torque multiplication, ignoring drivetrain losses, equals the overall ratio — a higher ratio means more torque at the wheels but a lower top speed for a given engine RPM.
Worked example
3000 RPM, a 1.00 transmission ratio (direct/top gear), a 3.73 final-drive ratio, and a 28 in tire: overall ratio = 3.73, wheel RPM ≈ 804, and vehicle speed ≈ 67 mph (π × 28 in × 804 RPM × 60 min/hr ÷ 63,360 in/mile).
Assumptions
- No tire slip, and 100% drivetrain efficiency (torque multiplication is the theoretical maximum).
- Tire diameter is the loaded rolling diameter, not just the sidewall-stamped size.
- Transfer-case ratio defaults to 1.00 (not applicable) unless changed in Advanced.
Reference
Gear ratio and drivetrain fundamentals: Wikipedia — Gear Train.
Related tools
Frequently asked questions
How do I calculate overall gear ratio?
Multiply the transmission ratio by the final-drive ratio, and by the transfer-case ratio if present.
How does tire diameter affect speed?
A larger tire increases speed at the same wheel RPM (effectively lowering the gearing); a smaller tire does the opposite.
Why does my real-world speed differ from the calculated value?
Tire slip, drivetrain efficiency, load, tire pressure, and speedometer calibration all introduce real-world variation.