Gear ratio and road speed calculator
Wheel revolutions per engine revolution is the product of the gear and final drive ratios, and everything else follows.
rpm
Road speed
90.50km/h
- In miles per hour
- 56.23
- Engine speed at the target
- 3315 rpm
- Overall ratio
- 3.3150
- Wheel speed
- 754.1 rpm
- km/h per 1000 rpm
- 36.199
Show the working
- Engine speed = 2500
- Gearbox ratio = 0.85
- Final drive ratio = 3.9
- Tyre rolling circumference = 2000
- Target road speed = 120
- Overall ratio = 0.85 * 3.9 = 3.315
- Wheel speed = 3.315 > 0 ? 2500 / 3.315 : 0 = 754.148
- speed_ms = 754.148 * 2000 / 1000 / 60 = 25.1383
- Road speed = 25.1383 * 3.6 = 90.4977
- In miles per hour = 90.4977 / 1.609344 = 56.2327
- Engine speed at the target = 2000 > 0 ? 120 / 3.6 * 60 * 1000 / 2000 * 3.315 : 0 = 3315
- km/h per 1000 rpm = 2500 > 0 ? 90.4977 / 2500 * 1000 : 0 = 36.1991
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- wheel rpm = engine rpm / (gear ratio x final drive)
- speed = wheel rpm x rolling circumference
A numerically higher final drive means more torque multiplication and a lower top speed in each gear.