Calculators · Tyres and gearing
Gear ratio, road speed and wheel torque calculator
The gearbox ratio, final drive and tyre size together decide how engine speed turns into road speed and how engine torque is multiplied at the wheels. These calculators give road speed at any RPM, the RPM needed for a given speed, and the overall reduction and wheel torque — the basis for choosing axle ratios, checking cruising RPM or working out crawl ratios for off-road vehicles.
Road speed at a given RPM
Calculates vehicle speed from engine speed, the selected gear, the final drive and the tyre diameter. It ignores tyre slip and load deflection, so real speeds are usually a little lower.
wheel rpm = engine rpm / (gear × final) speed [km/h] = wheel rpm × π × D [m] × 60 / 1,000
Engine RPM at a given road speed
Works out how fast the engine turns at a cruising speed. Use it to see the effect of a different final drive or tyre size on motorway RPM and fuel consumption.
engine rpm = speed × 1,000 / 60 / (π × D) × gear × final
Overall ratio and wheel torque
Multiplies every reduction in the drivetrain — gear, transfer case and final drive — and applies it to engine torque, allowing for drivetrain losses. In a four-wheel drive’s lowest gear this overall figure is its crawl ratio.
overall = gear × transfer × final wheel torque = engine torque × overall × efficiency
Worked example: cruising RPM in top gear
- Tyre 225/45R17: diameter 634.3 mm, circumference 1.993 m.
- In a 1.00 gear with a 3.90 final drive, 3,000 rpm gives 3,000 / 3.9 = 769 wheel rpm.
- Speed: 769 × 1.993 m × 60 / 1,000 = 92 km/h.
- At 100 km/h the engine turns about 3,262 rpm.
- In a 3.50 first gear with a 2.72 low range, the overall ratio is 37.1 : 1.
Typical ratios
| Application | Final drive | Overall ratio in first gear |
|---|---|---|
| Economy car | about 3.0 – 4.3 | about 11 – 16 : 1 |
| Sports car | about 3.2 – 4.1 | about 10 – 14 : 1 |
| Pickup truck | about 3.2 – 4.1 | about 13 – 20 : 1 |
| Off-road vehicle in low range | about 3.5 – 4.9 | about 40 – 100 : 1 |
Related
About these results
Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.
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