Gearbox Ratio to RPM Calculator
Calculate RPM from gearbox ratios, gear ratios, and vehicle speed.
inches
1st gear: 2.5-4.0, Top gear: 0.7-1.0
Engine RPM = (Speed x 5280 / Tire Circumference / 60) x Gear Ratio x Axle Ratio
Example: 60 mph with 28 inch tires, 1.0 gear ratio, 3.73 axle ratio = 2,823 RPM. At 3,000 RPM you would be doing 64 mph
How do I find my gear ratios?
Check your vehicle's manual, look up specs online by VIN, or check the RPO codes on GM vehicles. Axle ratio is often on a tag on the differential.
What's the formula for calculating RPM from speed?
RPM = (Speed x Gear Ratio x Axle Ratio x 336) / Tire Diameter. Or: RPM = (MPH x 5280) / (Tire Circumference x 60) x Overall Ratio
Why does tire size affect RPM?
Larger tires cover more distance per revolution, so the engine turns fewer times to travel the same distance, resulting in lower RPM at any given speed.
📐 Formula
Engine RPM = (Speed x 5280 / Tire Circumference / 60) x Gear Ratio x Axle Ratio
📝 Example Calculation
Example: 60 mph with 28 inch tires, 1.0 gear ratio, 3.73 axle ratio = 2,823 RPM. At 3,000 RPM you would be doing 64 mph
❓ Frequently Asked Questions
How do I find my gear ratios?▼
Check your vehicle's manual, look up specs online by VIN, or check the RPO codes on GM vehicles. Axle ratio is often on a tag on the differential.
What's the formula for calculating RPM from speed?▼
RPM = (Speed x Gear Ratio x Axle Ratio x 336) / Tire Diameter. Or: RPM = (MPH x 5280) / (Tire Circumference x 60) x Overall Ratio
Why does tire size affect RPM?▼
Larger tires cover more distance per revolution, so the engine turns fewer times to travel the same distance, resulting in lower RPM at any given speed.