Up or down – 4×4 tyre pressure guide

Staff Writer

Every vehicle owner should have a basic understanding of the importance of running the correct air pressures in their vehicle’s tyres for on-road driving, no matter what type of vehicle they drive.

The correct tyre pressures ensure optimum vehicle performance in terms of traction, grip, ride quality and fuel economy. It also promotes optimal tyre life in terms of wear across the tread face, as well as sidewall longevity.

For off-road driving the correct tyre pressure is of even greater importance. Adjusting air pressure to suit different terrain types will not only improve vehicle performance and ride quality while prolonging tyre life, but it will also minimise track damage.

While many four-wheel drivers are well-versed in the need to lower tyre pressures for sand driving, many don’t understand the benefits of lowering pressures for driving on other surfaces such as gravel roads and dirt tracks, or through mud and over rocks.

Before we go any further, however, it should be noted that the recommended pressures listed in this guide are just that, recommendations. It should also be noted that these recommendations are for vehicles fitted with Light Truck (LT) construction tyres, which have a stronger carcass – both across the tread face and in the sidewalls – than the Passenger Car (P) rated tyres that are often fitted as standard on many new 4x4s.

Also bear in mind that heavily laden vehicles will need to run higher air pressures in their tyres than lightly laden vehicles.

Road pressures

To establish a starting point when it comes to setting the correct tyre pressures for on-road driving, consult your vehicle’s tyre placard inside the driver’s door. Bear in mind, however, that while this shows different tyre pressure recommendations for different OE tyre sizes, you might have to adjust pressures to suit aftermarket LT tyres if fitted. The placard will also likely list higher pressures for when the vehicle is carrying a load, and this is an important consideration, especially for 4×4 bakkies that have high payload capacities.

If a vehicle is carrying a heavy load, it might be necessary to run higher pressures in the rear tyres than the front. Likewise, a vehicle equipped with a steel bullbar, a winch and driving lights will probably need higher tyre pressures up front than what is listed.

Figuring out the best on-road tyre pressures to suit your vehicle is often a case of trial and error, and it will depend on how you want your vehicle to behave on the road in terms of ride quality, steering response and handling.

It is important not to go too extreme when adjusting tyre pressures for on-road driving. Go too low and the tyres will flex a lot in the sidewalls and generate too much heat, which can result in tyre delamination. Go too high and ride quality will suffer through a lack of sidewall flex, and the tyres will also be more prone to punctures.

Pressures also have a big effect on tyre life. Overinflated tyres will wear prematurely in the centre of the tread area, whereas underinflated tyres will wear more on the outside of the tread area. With the right inflation pressures, the tyres should wear evenly across the tread face, assuming the wheels are correctly aligned and balanced.

Gravel and dirt roads

Driving on gravel roads and dirt tracks is very different to driving on tarred roads, so it’s important to adjust tyre pressures to suit the conditions.

For smooth and well-graded gravel roads on which you can easily maintain highway speeds, you might be able to stick to your on-road tyre pressures, but the loose surface will mean you have reduced traction and grip, resulting in compromised cornering capability as well as extended braking distances. Lowering tyre pressures results in a longer tyre footprint for improved traction and grip, and more flexible sidewalls for better ride quality.

As a general guide, if you run 240kpa on the road you might want to go down to 200kpa on smooth gravel roads. If the road surface deteriorates, you can lower pressures further.

I’d suggest somewhere between 220kpa to as low as 180kpa for rough gravel roads, depending on the load you are carrying. If for example, your on-road tyre pressures were at 260kpa, drop to 220kpa or so on rough gravel and see how the vehicle rides and handles. If you are running 240kpa on the road, then maybe drop to around 180kpa on rough gravel roads.

The improved ride quality you’ll experience by lowering tyre pressures is beneficial for the comfort of vehicle occupants and it makes driving less tiring. Lower pressures also minimise potential vehicle damage caused by incessant vibrations, and result in more grip, making the vehicle easier to drive.

As well as offering better traction, grip and ride quality, lowering tyre pressures for gravel roads also reduces the risk of chipping across the tyre tread face, as the tyres are more flexible and therefore less prone to impacts from sharp stones and rocks.

Rocky terrain

Reducing tyre pressures in rocky terrain allows the tyres to flex, which means they can better conform to uneven terrain, thus improving traction and reducing the chance of tyre damage. As mentioned, lower tyre pressures also result in a longer tyre footprint, which means better traction.

If you’re crawling over rocky terrain in low range, you can lower tyre pressures quite a lot, as heat generation from high-speed driving will not be an issue. If you run LT tyres, 150kpa is a good starting point for rock crawling, but make sure your throttle and steering inputs are gentle and well-considered, because abrupt acceleration or steering inputs can result in the tyres slipping on their rims, which can lead to deflation, or the tyres coming off the rims altogether.

There is a downside to lowering tyre pressures in rocky terrain though, and that is increased susceptibility to sidewall damage. As you lower pressures, the tyre sidewalls bulge outward, so make sure you keep an eye out for sharp rocks or protruding tree roots.

As your speed picks up, so too will tyre temperatures, and therefore the risk of tyre damage, so remember to reinflate the tyres once track conditions improve!

Mud terrain

The right tyre pressures for mud driving depends on what type of mud you’re driving in. If it’s slippery and slimy on the surface, but you can feel a firm base underneath, then gravel-road tyre pressures (180 to 210kpa) will allow the tyres to cut through the top layer and hopefully gain traction on the firm base.

If the mud is deep and gooey, and you can’t feel a firm base at all, dropping tyre pressures to as low as 150kpa will hopefully give them a chance of gaining traction by creating a longer tyre footprint.

As with rock driving, if you have lowered tyre pressures significantly to get through mud, don’t make sudden throttle or steering inputs, as the tyre could slip on the rim or mud could work its way in between the bead and the rim, resulting in rapid tyre deflation.

Sand terrain

Lowering tyre pressures for sand driving seems to be the one everyone knows about, but how low should you go, and why?

As already mentioned, lowering pressures increases the tyre footprint, and in sand this helps the tyres to float over the surface rather than dig into it. So, how low should you go? In soft sand, you can safely drop tyre pressures to as low as 110kpa without peeling them off their rims, so long as you don’t make sudden steering movements or corner at great speeds. And remember, don’t drive too fast on sand with low pressures as heat will quickly build up and cause tyre damage.

On harder-packed sand you might not have to lower pressures as much. You’ll figure it out as you go along by judging how easy or difficult it is to maintain forward progress. If it’s relatively easy, don’t drop pressures too much, but if it feels like a hard slog, go lower.

Dropping tyre pressures in sand not only reduces the chance of getting bogged, but it also reduces the strain on your vehicle and minimises the occurrence of track damage.

Tips and tricks for driving on sand dunes

The thrill of conquering a massive dune in a 4WD is hard to beat but be aware of the golden rules of dune bashing.

It’s all fun and games frolicking around in the sand, but make sure you reinflate your tyres to on-road pressures once you’re back on the blacktop. While you’re pumping them up, give them a proper once-over to see if there are any signs of damage, in which case you’ll have to repair or replace. Check the valve stems for leaks too, and make sure that the dust caps are screwed back on once the tyres have been reinflated.

If you only have a short distance of tar before you’ll be back on gravel again, you can maintain lower tyre pressures, but keep your speed down so your tyres don’t overheat.

Final thoughts

When I think about 20-something me asking: “What on earth are you going to do?,” or the air referee at the Toyota Rust de Winter 4×4 Jamboree saying: “180kpa will be just fine”, I don’t pass judgement.

Heading into the bush, whether for the first time or hundredth time, there is always a new skill to learn or improve upon. I’ll be heading to Kgalagadi next month, followed by a short stint to Mozambique and then 10 days in the Delta, nearly all on the dirt and aired down. I can’t wait to see what these next adventures teach me. See you in the bush!

Required gear

This is the gear you’ll need to regularly adjust tyre pressures to suit off-road terrain, and you should have in your kit in case of tyre damage:

• Quality tyre pressure gauge