Fitting big chunky off-road tyres will no doubt take your 4WD through the rough stuff easier and give you that aggressive look as well. However, there are some other factors that ultimately affect your off-road ability.

Changing the size of your tyres impacts everything in the driveline by allowing all parts to rotate at a different speed. Your 4WD comes from the factory with the optimum gear ratio to work with the engine, transmission and stock tyre size. When larger-diameter tyres are installed with original diff ratios, the engine will operate at a lower RPM. This results in reduced performance and suffering fuel economy from the engine not operating in its peak power band. To make matters worse, your speedo will no longer be indicating your actual road speed. This is because it was calibrated to the rolling diameter of your fourby's original tyres and it is unaware anything has changed.

One of the last things most 4WD owners wish for is more frequent stops at the fuel pump. Being that most 4WDs are about as aerodynamic as a brick, we looked into how the size of your tyres will effect fuel consumption.

Previously, we dyno-tested a GU Patrol boasting a 4.8L petrol engine with standard 31in tyres. These figures were then compared to a new test using 35in tyres. The results indicated a drop in the GU's power output by a staggering 13%, which will result in more right boot to get your fourby up to cruising speed.

Given that Australia has so much terrain to cover, these hurdles need to be removed. The good news is there are solutions to these problems that will restore some of your lost efficiency and get your speedo back into gear.

AVOID BIND-UP

Both front and rear differentials will require an equal ratio change so they turn at the same speed and avoid bind-up, which causes costly damage to the weakest link in the driveline.

The Patrol's most likely candidate for failure is the front hubs followed by axles and universal joints. If you own a 'Cruiser, expect differential failure. The front diff in particular will be the 'Cruiser's weak point.

When your front and rear ratios are the same, these mechanical nightmares will no longer be a side effect.

RATIOS TO SUIT TYRES

Altering your differential ratios to suit the increased tyre size will ensure the engine continues to deliver its usual efficiency, which means less fuel fills during your trip. There are many ratios to choose from, but the closer the new ratio is matched to your new tyre size, the less of a negative impact your tyres will have on your fourby.

One of the side effects is increased load on the drivetrain. Once again, this is a direct result of increasing tyre size. The force required to rotate a larger-diameter tyre is greater, and this in turn places more load on the engine and added stress on the drivetrain.

The general rule of thumb is to go up a ratio for every tyre size. For example, if you were to change your diesel Patrol's tyre size from a standard 31in to a 33in, you will also need to increase the factory gear ratio of 4.11:1 to 4.37:1. By using this method, you will maintain the same amount of power and torque. There is no point having large, go-anywhere tyres when your engine will no longer take them places. Included is a gearing calculator that indicates the correct gear ratio option to suit the many different tyre sizes on the market.

Here is a formula that will let you calculate your RPM at 104km/h (typical highway speed).