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How It Works
The gear equation links vehicle speed to engine RPM, tire size, and the final drive gear ratio. The constant 168 handles the unit conversion from inches and RPM to miles per hour (63,360 in/mi ÷ 60 min/hr ÷ 2π). Solve for any one variable when you know the other three.
Example Problem
A truck with 15-inch tire radius, 3.73 gear ratio, cruising at 3,000 RPM. What is the speed?
- Speed = (15 × 3,000) / (168 × 3.73)
- Speed = 45,000 / 626.64 = 71.8 mph
Frequently Asked Questions
What happens if I install larger tires without re-gearing?
Larger tires lower your engine RPM at highway speed, which can hurt low-end acceleration and cause the transmission to hunt between gears. Re-gearing to a numerically higher ratio (e.g., 3.73 to 4.10) restores the original RPM range.
What is a good gear ratio for fuel economy?
Lower ratios like 2.73:1 or 3.08:1 keep RPM down at highway speed, improving fuel economy. However, they sacrifice acceleration. The best ratio depends on tire size and the engine's torque curve.
Does this formula account for transmission gears?
The gear ratio here is the overall ratio (transmission gear × differential ratio). In top gear (often 1:1 or an overdrive like 0.7:1), the effective ratio equals the differential ratio multiplied by the transmission gear.
Related Calculators
- Horsepower Calculator — HP, torque, and RPM.
- Engine Equations Calculator — volumetric efficiency and displacement.
- Torque Calculator — calculate torque from force and lever arm.
- Tire Size Comparison Calculator — compare tire dimensions that affect final drive ratio.
- Speed Unit Converter — convert between mph, km/h, m/s, and other speed units.
Note: Results are approximate and assume no drivetrain losses or tire slip.