Cruise rpm, ratio and speedo error
Enter a tyre size like 265/75R16, or an overall diameter in inches. Gearbox ratio is 1.00 for a direct top gear, or the overdrive ratio for the gear you cruise in.
| Speed | RPM |
|---|
Estimates only, based on rolling diameter. Tyre wear, pressure and brand vary. For a build we will confirm the numbers on the car.
How gear ratio works
Diff ratio is the number of turns of the pinion for one turn of the wheels. A 3.73:1 ratio means the tail shaft turns 3.73 times for every turn of the wheel. A numerically higher ratio (say 4.11) gives more acceleration and higher cruising rpm; a lower ratio (say 3.08) gives a quieter, more economical highway car.
Fitting bigger tyres raises the effective ratio and dulls the car. Going from a 30 inch to a 35 inch tyre changes the final drive by around 17 percent, so a 3.07 would want roughly a 3.55 to bring the feel back. Bigger tyres also make the speedo read slow and the odometer under-read.