Final Drive / Diff Swap Calculator

See how a new differential ratio changes RPM at speed and acceleration.

How it works

1

Enter both ratios

Your current diff ratio and the one you are considering.

2

Add a reference RPM

Engine RPM at a known cruising speed with the current diff.

3

See the change

A numerically higher ratio raises RPM and acceleration but also revs and consumption.

Frequently asked questions

What does a higher final drive ratio do?

A numerically higher ratio (e.g. 3.90 vs 3.45) multiplies torque to the wheels, so acceleration improves, but engine RPM at any given speed rises by the same percentage, raising cruising revs and fuel use.

How much faster is a diff swap?

Acceleration scales roughly with the ratio change. Going from 3.45 to 3.90 is about 13% more wheel torque, which gives a noticeable in-gear gain without touching engine power.

Will a diff swap change my speedometer?

Only if the speed signal is taken before the diff; most are taken at the wheel, so the speedo stays correct, but the rev limiter is reached at a lower road speed in each gear.

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