Gear & Diff Ratios

When we talk about the diff ratio, what do we mean?

To understand diff ratios, you need to understand the physics of leverage. You have a heavy load to lift, so you use a long lever positioned on a pivot with the longest part of the lever between the pivot and the application of force. The smaller portion of the lever is between the pivot and the load. The longer the lever is on the force application side, the less you will move the load, but also easier it will be to lift.

The final drive (differential) ratio is measured by dividing the number of teeth on the crown-wheel, (driven) gear, by the number of teeth on the pinion (drive) gear drive. In most instances the driven gear will always have more teeth than the drive gear.


What will the final drive ratio affect? Your Speedometer Reading!

Because rolling circumference determines how far the wheel will travel, the diameter of the wheel and tyre size will affect the speedo reading. A different differential gear-set ratio will also affect the speedo reading.


It is important to match the final drive (differential) ratio with both the engine power and the weight of the vehicle (loaded).


» Download our Diff Lapping Ratio Guide

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