Gear Ratio RPM Calculator
3000 engine RPM through a 3.73 rear gear = 804 wheel RPM — add tire size for road speed.
Gear ratio is the rear-axle (ring-and-pinion) ratio in direct drive. Road-speed figures assume no torque-converter slip or tire deformation — real-world speed varies slightly.
How the gear ratio calculator works
A 3.73:1 gear ratio means the driveshaft turns 3.73 times for every single turn of the wheels. Divide engine RPM — measured in direct drive, where the transmission ratio is 1:1 — by the gear ratio and you get wheel RPM, the raw number everything else is built from.
Road speed comes from wheel RPM times tire circumference: each wheel revolution moves the car forward one tire-circumference, so miles per hour is wheel RPM × π × tire diameter in inches, scaled from inches per minute to miles per hour.
wheel RPM = engine RPM ÷ ratio · mph = wheel RPM × π × tire in × 60 ÷ 63360
Related tools
Gear math is all ratios and conversions — our Decimal to Fraction Calculator and Equation of a Line Calculator handle the rest of the garage arithmetic.
Gear ratio RPM calculator FAQ
What does a 3.73 gear ratio mean?
The driveshaft (pinion) turns 3.73 times for each single turn of the wheels.
Higher vs lower gear ratio — which is which?
A higher number (e.g. 4.10) is a "lower" gear: more torque and quicker acceleration, lower top speed; a lower number (e.g. 3.08) cruises at lower RPM.
Does tire size change my effective ratio?
Yes — taller tires act like a numerically lower gear (fewer effective RPM per mph).
Why does my speedometer change with bigger tires?
Most speedometers read from transmission/driveshaft output, so each wheel revolution now covers more ground and the speedo reads low.