How to calculate caravan tyre pressure

Travel & Touring | Camping & Caravanning

15 November, 2024

They heat up when you drive and so the pressure rises. They cool down when you stop and so the pressure drops. Air leaks out of imperfections or miniscule gaps and so over time, they deflate.

This constant flux, combined with the fact caravans aren't used every day, means that checking the pressure before each trip is really important. It's very easy to tow with underinflated tyres.

But what is the correct pressure for your caravan's tyres? No doubt we're all guilty of just inflating the tyres to whatever setting the last person at the service station left the air pump at. But that may not be good enough for your caravan tyres which could be under significant load and have different requirements to your car.

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The risk of underinflated tyres

It's more than likely that if you haven't checked your caravan's tyre pressures in a while, they're underinflated. Tyres of all kinds naturally deflate as the molecules of air find their way through molecular-sized holes in the rubber.

They can also lose pressure through imperfections in the seal between wheel and tyre, or if the valve is old or stuck open. Many punctures only let out air very slowly, and so you may not notice it until you check on your van a month after your last road trip. There may not even be a leak, but the tyre gauge you are relying on may be inaccurate.

Underinflated tyres are more susceptible to a few common issues. If absolutely nothing goes wrong, you'll at least shorten the life of your tyres thanks to premature or uneven wear. Underinflated tyres tend to wear out the sides of the tread faster and create more rolling resistance and therefore worsen fuel economy.

Heavily underinflated tyres can damage the sidewalls which are forced to flex more. That increased flex can cause them to overheat and blowout or rupture.

Wear and tear aren’t the only problems caused by underinflation. A large part of the tyre's load capacity is tied to its inflation. For example, if a tyre is rated to carry 500kg, it can only do so at a pre-determined pressure. If it's got less air in it, the load carrying capacity of the tyre is reduced.

The risk of overinflated tyres

Of course it's also possible to overinflate tyres. If that's the case, the tread can balloon out, reducing the tyre's contact patch on the road. That often leads to increased wear in the centre of the tyre and a reduction in braking performance.

Overinflated tyres may also be too stiff, and so reduce the effectiveness of your caravan's suspension, possibly leading to premature wear of other components, or a broken glass or two.

Overinflated caravan tyres are also more likely to puncture, much like an overinflated balloon is easier to burst. They're less likely to bend over imperfections in the road, like potholes or debris, so can flatten suddenly.

Calculating tyre pressure for correct inflation

Check the caravan's tyre placard or compliance plate

The easiest way to find out what the correct inflation should be is to read the tyre placard, if your caravan has one. Most do, and all modern caravans should.

Commonly the tyre information in etched into the compliance plate of the caravan, which is usually found on the draw bar or in the front boot.

As well as information like the aggregate trailer mass and other important information, many VIN plates, particularly those supplied by the Caravan Industry Association of Australia (they have a blue key logo with RVMAP written inside it), have fields for tyre information. They will specify the recommended tyre pressure with no load and maximum load in kPa (kilopascals, the metric equivalent of PSI - pounds per square inch).

This information is often taken from tyre manufacturer Load Inflation Tables, which are usually large, dense tables that detail how much load the manufacturer's tyres can handle at different pressures. For example, on Bridgestone's Light Truck Load Inflation Table, you can see that for a 265/75R16 LT tyre (like a Dueller AT), it can carry 890kg at 250kPa (35psi) or 1120kg at 350kPa (50psi). It goes all the way to 1550kg at 550kPa (80psi) for some versions. You may be able to Google the chart for your tyres, but if not, contact the store you bought the tyres from or a local dealer for that brand.

If you're mathematically inclined, you will notice that the relationship between load and pressure is not linier. Using the above example, 35psi is 70 per cent of 50 psi. But 890kg of load capacity is 79.5 per cent of 1120kg. This may not be the case for all tyres, but it stands as a warning against assuming that if your load is 85 percent of the tyre's maximum load, you can inflate them to 85 percent of the maximum pressure. That may be the case, but not always.

Check the load on your tyres

With this information, you now just need to work out how much load is on each tyre. There's a lot of benefit to having your caravan weighed once it's ready for travel, if only because evidence suggests the majority of caravanners don't know how much their van weighs and usually underestimate it. You can get your car and caravan weighed with Mobile Vehicle Weighing WA and save with your RAC member benefits. With an accurate weight, you can work out the load on each tyre and then inflate each one to the recommended pressure.

Your caravan has weight on its tyres and the draw bar, so you want to find out the gross trailer mass, and then divide that by how many wheels your caravan has.

What may surprise you is how much pressure your caravan tyres need. Most cars have somewhere between 32-28psi in each tyre. Many caravans could have as much as 70psi to deal with the load. For example, a single axle caravan with a gross trailer mass of 2200kg will have 1100kg of load on each tyre. Using the example of the Bridgestone tyres above, it would need around 50psi of pressure in each tyre to handle the load.

If you can't find the specific load information for your tyre, all tyres have a load rating marked on them. It will likely be after the tyre size info, and a three digit number followed by a letter. 121S for example is load rating 121, speed rating S. It may also be two three-digit numbers separated by a forward slash, for example 121/119S.

In any case, the first digit is most likely the relevant one, unless you have a motorhome or light truck which has dual wheels on the same axle.

Tyre load ratings are easy to Google - a tyre with a 121 load rating can carry 1450kg at maximum pressure, which will also be marked somewhere on the tyre. Using this information, you can, if you have no better guide, make an approximation of the pressure needed. If your load on each tyre is 70 per cent of the maximum load, around 70 percent of the maximum pressure should be close to correct - close enough to keep you out of trouble in the short term, at least.

As an example, if you have 1100kg of load on each tyre which has a 121 load rating and maximum pressure of 80psi, use this calculation: 

Actual tyre load divided by maximum tyre load = percentage tyre load - eg 1100 / 1450 = 76%. 

Then

Maximum tyre pressure times percentage tyre load as a decimal - eg 80 x 0.76 = 61 psi.  

How often should you check caravan tyre pressure?

Many tyre manufacturers recommend checking tyre pressures every two weeks to a month. As we often don't tow our caravans that often, check at least before every trip, and again once as you leave camp if you've been there for more than a fortnight.

The 4psi rule

If you read enough forums, you'll come across people mentioning the 4 psi rule. Even some tyre sellers recommend it on their websites.

The rule is based on the generality that if a tyre is properly inflated when cold, its pressure should increase by 4psi for passenger tyres (or 6psi for light truck tyres) under normal driving conditions on bitumen. If it increases more than 4psi (or 6psi), there wasn't enough air in the tyres. Wait for them to cool back down and pump them up a bit. If it rises less than 4psi, there was too much pressure in the tyres. Wait for them to cool down, deflate them a little and retest.

Not all tyre manufacturers recommend this, however, so the best advice is still to ask the tyre manufacturer or retailer for the load pressure chart.

However, the 4psi rule does offer some useful observations. The temperature of a tyre can be a good indication of incorrect tyre pressure. Almost certainly if the sidewall feels hot to touch after normal driving, it's probably underinflated. If it's cool, it may well be overinflated.

Recommended tools

If you are serious about keeping your tyres at the right pressure and in great condition, you should carry a few inexpensive tools and spares. Don't forget you can save on a range of products at Repco with your RAC member benefits.

Tyre pressure gauge

Most service stations have compressors to inflate tyres which include a gauge. But it's a good chance that gauge is inaccurate, especially if it's attached to the hose and has likely been dropped on the concrete a few times. Good quality tyre gauges are inexpensive (but avoid the cheapest of them) and available at most camping or auto stores. Keep it in the glove box or door pocket so it's always at hand.

Valve key and spare valves

Valve keys are small multi-tools that help remove and replace tyre valves, as well as being better than a twig for deflating tyres. Keep a key and a couple of spare valves in case you get a leak from one.

12-volt compressor

Particularly if you do more remote camping, a 12-volt air compressor gives you the ability to pump up your tyres anywhere you need to. They're almost essential if you travel off-road, as you'll need to reduce tyre pressures for rougher-roads and them pump them up again once you're on bitumen.

Tyre pressure monitoring systems

Tyre pressure monitoring systems don't fall into the inexpensive tools category, but they can be really helpful in maintaining correct pressure. Usually you’ll replace your valve-caps with Bluetooth enabled pressure-sensors that send info to a small in-car display, so you know if a tyre is overheating (pressure rising more than the other tyres), or deflating. They're an early warning system that can help you prevent worse tyre damage.