Australian 4WD Action – July 2019

WHAT HAPPENS WHEN YOU

FIT LARGER TYRES

CALCULATING WHICH

RATIOS YOU NEED

Firstly, the larger rolling diameter throws your speedo out of

whack because your speedo is calibrated from the factory

taking into account every gear ratio in your vehicle - from your

gearbox to your transfer, diffs and tyres. Changing any one of

these will affect the others. A larger set of rubber makes your

speedo read slower than your actual speed because you are

travelling a further distance per full tyre rotation. This means

that, the greater the increase in tyre size, the more your

speedo will be out.

It also plays havoc with your gearing. Each gear is now

taller, you’ll find your 4WD doesn’t have as much pull as it did

beforehand, it’s slower to get up to speed and up hills you may

find you need to drop an extra gear or two to keep your speed

up. Off-road it gets worse. Controlled crawling of slow

technical tracks becomes a bit hairier because the taller

gearing sees you hit it at a bit more speed.

The good news is, all of this can be corrected simply by

changing your diff gear ratios.

Before changing diff ratios, you need to know what ratio you

have and then you can figure out what you need to change to, to

restore your factory final drive ratios.

Here’s how to work it out:

‘Current Gear Ratio x New Tyre Diameter (mm) ÷ Factory Tyre

Diameter (mm)’.

Using a GQ Patrol with a new set of 35s as an example, that’s

4.1 x 884mm ÷ 804mm = 4.5.

So a 4.5:1 diff ratio would bring the gearing back to standard

spec with 35in tyres. The closest option for the Patrol is 4.6:1

gears, which come factory fitted to the RD28T GQ and early GUs,

but are also sold brand new through aftermarket suppliers.

Some owners opt for somewhere in between, often opting

for a 4.3:1 ratio.

WHAT ARE DIFF RATIOS?

Your diff ratio is the number of turns of the pinion compared to

the number of turns of the crown wheel. Or more simply, the

number of tailshaft rotates it takes to complete one full tyre

rotation. For example, if you have diff ratios of 4.3:1 - your

pinion will turn 4.3 times for every 1 full crown wheel rotation.

This difference is achieved by changing the number of teeth

(and ultimately the diameter) of the crown wheel and pinion

gear set, just like changing sprockets on your old push bike.

Most 4WDs will have a model number or identification plate

on the firewall or on the housing. It will read diff model then

have a code. Patrols have model numbers like HG41 or HH43.

The number refers to the diff model and the number refers to

the ratio, so the 43 will be a ratio of 4.3:1 and so on.

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