DIFF RATIO BASICS

The ‘diff ratio’ (or ‘final-drive ratio’) is effectively a measure of how many times the pinion gear must turn — thus turning the crown wheel — in order to turn the driven wheels one complete rotation. So, if a diff ratio is 3.08:1, we know that the pinion gear must rotate 3.08 times for each full rotation of the crown wheel.

The easiest way to get an idea of diff ratio, when the diff is still in the car, is to mark a line on the pinion and diff housing, and a line on the tyre and a constant surface (such as the guard), or the hub and a constant surface (such as the brake caliper). With the transmission in neutral, spin the driveshaft — which is directly attached to the pinion gear — and count the number of rotations it makes before the wheel or hub has completed a full rotation. This is your final-drive ratio.

Changing the final-drive ratio can affect the vehicle’s driving characteristics. A higher ratio, such as 3.9:1, means that the driveshaft must complete more rotations for each rotation of the wheels, meaning that the engine must rev higher to move the wheels the same distance. The advantage is quicker acceleration, while the disadvantage is higher cruising rpm and a lower top speed. A car at the drag strip with 4.11 gears and a 224kW engine will launch harder and will be quicker than a car that has 373kW and gears that are way too tall. Moving to, say, a 2.79:1 ratio would have an advantage of a lower engine speed while cruising, but the trade-off would be slower acceleration off the mark.

WORKING WITH NUMBERS

You can calculate your ‘ideal’ final-drive ratio through a formula, if you know a few criteria: speed (mph), desired rpm (at that speed), and tyre diameter:

Say your car runs on 245/35R18 tyres, and you’d like its motorway cruising speed of 100kph (62mph) to be at no more than 3000rpm. We are assuming a top gear ratio of 1:1, as the formula doesn’t factor for overdriven ratios.

First off, we need to calculate tyre diameter: 18-inch wheels mean the 18-inch centre diameter is a given, so we need to calculate sidewall height.

The 245/35 means that the tyres are 245mm wide, with a sidewall height of 35 per cent of the width. Thirty five per cent of 245mm is 85.75mm, which is the sidewall height.

To calculate tyre diameter, we need to double this — if you look at the tyre side on, its height is made up of the sidewall at the bottom, the wheel diameter, and the sidewall at the top. So we really have sidewall height of 171.5mm.

This translates to 6.75 inches. Combining this and the diameter gives us a total tyre diameter of 24.75 inches.