If you’ve been wondering whether you are running the right tyre pressures on your 4WD when driving on a bitumen road, this post covers everything you need to know. We also cover a calculator that will give you the exact tyre pressure you should be running, regardless of what your tyre is, or how heavily you are loaded.
Why you should care about tyre pressures
4WD tyres are not cheap, and you’re wise to take a bit of time to make them last as long as possible. When you are paying up to $500 per corner for new 4WD tyres (especially as your wheel size increases), its a fair old whack to the hip pocket.
The right tyre pressures ensure your tyres last as long as possible, that they won’t fail due to getting too hot, that they break and corner correctly (and safely), and that your tyres wear evenly across the tread. Running the wrong tyre pressures will cause premature wear, early failure and possibly even terrible (and unsafe) vehicle handling.
When do you need more air pressure?
In general, the smaller the tyre, and the more load you carry the higher the air pressures need to be.
Look at your tyre placard
A good place to start when looking at 4WD tyre pressures is to check out the tyre placard on the A pillar of your drivers door (near where the seat belt bolts in). This will list the different tyres that are run on the vehicle you own, and the recommended cold inflation pressures. They’ll almost always be in kpa, which we should all be using, but everyone seems to stick with PSI in Australia!
Calculate the correct tyre pressures
If you want to get the correct tyre pressures to an absolute tee, you can do it really easily. You just need to read the data on your tyre sidewall, and know roughly how much weight is on each wheel. The easiest way to tell this is to use a weighbridge or mobile weighing service, or you can go off the vehicle manufacturers specifications and add in any weight that you’ve put on.
Nothing beats getting the vehicle weighed properly though, just be prepared for a bit of a shock!
The exact tyre pressure depends on a huge number of factors such as tyre size, tyre type, load on the tyre, what terrain you are driving on, how fast you are going and so on and so forth. There’s a number of theories that you can use (like the 4 or 6 PSI rule), but the easiest way to work out your 4WD’s bitumen tyre pressures is to follow the below formula.
Weight on the tyre divided by the tyres maximum load rating, then multiplied by the maximum cold pressure of the tyre.
Lets start right at the beginning, in a practical example. If you have had your vehicle weighed, you should know what load is being applied to each tyre. Our front axle weight on the Dmax is around 1300kg, which means we have 650kg on each tyre.
On the rear, we’re at about 1870kg, so 935kg of weight onto each tyre.
From there, you can head to the sidewall of your tyre, and you’ll see two figures; the maximum load rating, and the maximum cold pressure of the tyre. For our Dmax, running Toyo Open Country RT’s, its 1550kg, and 80 PSI.
From there, its just a case of doing the equation.
For the front, its 650/1550, then multiplied by 80 PSI. This gives us 34 PSI as a cold tyre pressure on the front axles.
For the rear, its 935/1550, then multiplied by 80 PSI. This gives us 48 PSI as a cold tyre pressure for bitumen driving.
Its that simple, and there really is nothing more to it. If you really want, pump your tyres to the recommended bitumen tyre pressure as per the above (when they are cold), and then drive for 30 minutes and check the increase. Ideally the tyres wont have gone up by more than 6 PSI, but this is subjective to the ambient temperature (and which side is getting sunlight) and isn’t always accurate.
Watch the tyre wear
The other very simple way to tell if you are running the right tyre pressures is to look at the tyre wear, at least every few thousand kilometres. If the centres of your tyres are wearing out, you are running pressures that are too higher.
If the outers are wearing out, you have too low tyre pressures. If just the inner, or outer on one tyre is wearing out, you have a wheel alignment issue.
It’s that simple.