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Gear Ratio / RPM / MPH Calculator
Calculate gear ratio from ring and pinion teeth
The gear ratio calculator converts between RPM, road speed, tire diameter and total gearing — transmission ratio times axle ratio.
Formula: MPH = RPM × tire diameter ÷ (336.14 × total gear) · RPM = MPH × 336.14 × total gear ÷ tire diameter
What this number means
This calculates a rear-end gear ratio from ring gear and pinion tooth counts, and works for transmission and primary ratios too. It removes the guesswork when a centre section has no tag or the tag cannot be trusted.
What to change next
Count teeth before ordering anything — a misidentified ratio throws off every gearing and speed calculation built on it. If the counted ratio is not what the car needs at the stripe, this confirms the swap is actually required.
Worked example
6,500 RPM on a 28 in tire through a 4.10 axle in high gear (1.00 transmission ratio) works out to ≈ 132 MPH.
Related calculators
Read the guide: Gear Selection Guide →
Building a Jeep or 4x4 too? The same gear math drives your crawl ratio. RegearReady — our sister site — turns it into regear decisions: Calculate your gear ratio on RegearReady.