Setting the correct air pressures in your tyres to suit different on- and off-road driving conditions is dependent on many factors. Here are some guidelines to point you in the right direction.
MOST FOUR-WHEEL DRIVERS know to drop the air pressures in their tyres for sand driving, but what about when driving through mud or over rocks, or even along corrugated gravel roads?
Adjusting the air pressure in your tyres to suit different driving conditions will not only improve your vehicle’s performance, but also aid tyre life, help prevent tyre and track damage, and result in a much more comfortable ride.
The suggested pressures listed here are for vehicles fitted with Light Truck (LT) construction tyres, which have a stronger carcass than Passenger Car (P) rated tyres. Also bear in mind that heavily laden vehicles will need to run higher air pressures in their tyres than lightly laden vehicles.
As a general rule you should follow the recommended tyre pressures as stated on your vehicle’s tyre placard for on-road driving which, for most 4×4 wagons and utes, will be between 30psi and 38psi. The tyre placard will usually state different pressure recommendations dependent on the tyre size and the load the vehicle is carrying; the higher the load the higher the tyre pressure.
For drivers of 4×4 utes, for example, this might mean running 32psi in all four tyres when the vehicle is unladen, but when laden running 32psi up front and 36psi in the rear.
Finding the ideal tyre pressures for your vehicle may take some time, and factors to help you arrive at the ideal settings will include preferred ride quality, steering response, vehicle handling and tyre wear. A tyre that’s been overinflated will wear prematurely in the centre of the tread area, whereas a tyre that’s been underinflated will exhibit more wear on the outside of the tread area. If you have the pressure right, the tyre should wear evenly across the tread.
Rough Gravel Roads
When you reach the end of the blacktop and you’ve got gravel under your wheels, it’s time to start thinking about your tyre pressures.
If the gravel road is smooth and well graded, and you’re maintaining a similar speed to when you were driving on sealed roads, you can leave your tyre pressures as they were. But if the road surface deteriorates and you have to lower your speed to suit the conditions, you should probably think about lowering your tyre pressures. By how much will depend on the load your vehicle is carrying and the speed you’re travelling at; the higher the load and the higher the speed, the more heat will be generated, so you don’t want to go too low.
Somewhere in the range of 26psi to 32psi is usually about right for rough gravel. If you were running 38psi on the road, then drop pressures to 32psi on rough gravel and see how it feels. If you were running 30psi on the road then 26psi might be appropriate on crook gravel roads and tracks.
Lowering your tyre pressures in these conditions will not only improve your vehicle’s ride quality, but also improve tyre flexibility, which can reduce the risk of chipping in the tread area.
Reducing your tyre pressure in rocky terrain has several benefits. Lower pressure allows the tyres to flex more easily so they can conform to the terrain, which improves traction and reduces the chance of damage from impacts, and it gives the tyres a longer footprint, which also aids traction.
So how low should you go? If you’ve engaged low-range to tackle rocky terrain and you’re driving at slow speeds, you can reduce your tyre pressures to as low as 22psi, but care has to be taken when driving at these pressures. It’s important to maintain controlled throttle and steering movements when driving with low tyre pressures to avert the risk of the tyres slipping on their rims, which could result in deflation or a tyre coming off the rim altogether. And don’t drive too fast, because excessive speed will generate too much heat, which can damage the tyres. If conditions improve and your speed picks up, inflate the tyres to around 28psi if the track is still rocky.
Finally, keep an eye out for nasty looking rocks and other sharp objects that could damage your tyres’ sidewalls, which will have ‘bagged-out’ due to the lower pressures.
Setting the correct air pressure for mud driving very much depends on one thing: the mud. If the mud is all slippery and slimy up top, but there’s a firm base underneath the surface, then relatively high air pressures are ideal. But if it’s all deep and gooey, with no firm base, then low air pressures will give the best result.
By running at close to road pressures of around 28psi, the tyres can cut through the top layer of mud and gain purchase on the firm base underneath, hopefully gaining enough traction to get you through the muddy sections. This is why it’s usually a good idea to stick to existing tracks when crossing a claypan, for example, as the tyres from the vehicles that have already been through will have cut through to that hard base layer under the surface.
When the mud seems bottomless, however, air pressure as low as 22psi will give your tyres a better chance of gaining purchase thanks to a longer tyre footprint. Once again, take care when driving with very low pressures as there’s always the chance of a tyre slipping on the rim, and mud it can work its way in between the bead and the rim, resulting in tyre deflation.
Dropping tyres pressures for sand driving is a given, but how low will depend on several factors, including how soft the sand is, how coarse it is, whether it’s wet or dry, and how hot the ambient temperature.
As mentioned, lowering air pressure increases the tyre’s footprint, which in the case of sand helps it to float over the surface rather than dig into it. If the sand is particularly soft, which is usually the case if its fine and dry, you can drop your tyres down to as low as 16psi without fear of pealing one off a rim, so long as you don’t drive too fast and you don’t make any sudden steering movements. If the sand is coarse and damp, it will usually be a little harder, in which case you won’t have to lower pressures as much.
As well as reducing the chance of getting bogged, lowering your tyre pressures in sand will reduce the strain on your vehicle’s engine and driveline components, and will minimise the chance of track damage.
The Final Word
All of the air pressure recommendations listed here should be used only as a general guide, as there are many variables such as track conditions, vehicle weights, tyre sizes and more. Whenever you lower your tyre pressures for off-road driving, remember that they will generate more heat, so speed and steering inputs have to be adjusted accordingly.
Finally, don’t forget to reinflate your tyres to the placard recommended pressures once you’re back on the road…and while you’re at it have a good look over each tyre to check for signs of damage, and repair or replace if necessary. Also check the valve stems for leaks and make sure the dust caps are in place.