## Tyre size and pressure are crucial for all wheels on your towing rig. Here’s how to calculate the correct tyre pressure for your caravan.

One of the most important safety aspects of towing is having the correct tyres fitted on your caravan and just as important having the tyres inflated to the correct pressure.

The simple fact is, caravan tyres often get overlooked and are not checked regularly and this can lead to all sorts of wear, handling and safety issues. On your tow vehicle, there is usually a sticker conveniently located in the door frame or fuel filler cap reminding you of the correct tyre pressure. Unfortunately, this is not always the case with caravans.

If you have one, your caravan handbook usually quotes the tyre pressure for the original tyre size fitted and the caravan loaded to the GTM. As the load in your caravan can change, or on older caravans, the tyre size might have changed, so it’s worth knowing how to calculate the correct pressure from the markings on the tyres fitted to your caravan.

You first need to be able to decipher the code on the tyre sidewall. This is a typical code: **205 / 65 R 15 94 H**

The first part of the number is the tread width, in this case, **205mm** wide.

The second number is the Aspect Ratio which means in this case, the tyre wall height is **65%** of the width of the tread.

The next letter **R** is the construction, in this case, Radial; the following number **15** is the rim diameter (in inches). The next number **94** is the load rating – more of this in a moment and finally the last letter **H** is the speed rating for the tyre.

### Load Rating

The load rating for the tyre can range from 60 to 130, which equates to a maximum load per tyre of between 250kg to 1900kg. The first thing to check is the tyres are rated for the gross trailer mass (GTM) of your caravan, so we need to know the load index ratings.

From the table above, you will be able to cross-reference the load rating code number to the actual maximum permissible load the tyre is rated for. So in our tyre data example from above, we can see that a load index of 94 means the maximum load for the tyre is 670kg. Remember that is the load for one tyre.

So on a single axle, those two tyres could carry a maximum load of 1340kg. The recommendation from the tyre industry though, is you should never exceed 90% of the tyre’s load index, so in this case, the maximum axle weight would be 1251kg.

The other piece of information we need to know from the tyre is the Maximum Tyre Inflation Pressure and Load. This is usually located on the sidewall below the tyre data in smaller text. It will say something like Maximum Load 720kg, Maximum Pressure 52 PSI, and it might have the pressure given in Bar. Once you have the GTM of your caravan and the maximum pressure for your tyre we can now work out the correct tyre pressure.

Take the figure for the maximum pressure for the tyre and divide it by the maximum weight from the table above. Now multiply this figure by the actual load you are going to put on the tyre – so half the GTM for a single-axle caravan or a quarter of the GTM for a twin-axle caravan. The resulting figure should be the correct inflation pressure for each tyre.

Let’s look at an example for a single-axle caravan:

Tyre Details: 205 / 65 R 15 94 H – Maximum Inflation Pressure 60 PSI

Weight of caravan: 1200kg

60 / 670 = 0.08955 (Max Pressure for tyre divided by load rating)

0.08955 x (1200 / 2) = 53.7 PSI (inflation factor times half the weight of the caravan)

So the correct tyre pressure is 54 PSI

Working out for a twin-axle caravan:

Tyre Details: 205 / 65 R 15 94 H – Maximum Inflation Pressure 60 PSI

Weight of caravan: 1800kg

60 / 670 = 0.08955 (Max Pressure for tyre divided by load rating)

0.08955 x (1800 / 4) = 40.29 PSI (inflation factor times one-quarter of the weight of the caravan)

So the correct tyre pressure is 41 PSI

### Top Tip

As a rule of thumb, the inflation pressure of a hot tyre is usually about 4 PSI higher than when cold. If you check your tyre pressures when hot and it is more than 4 PSI higher, then the cold inflation pressure was too low. If the hot inflation pressure is less than 4 PSI difference than the cold pressure, then the cold pressure was too high.

This is derived from: For every 10 Deg F temp change the tyre pressure changes by 1 PSI

This article is meant as guidance only. If you have any questions or are unsure what your tyre pressures should be, ask your caravan manufacturer or local dealership. If you have any doubts about the suitability or the condition of your caravan’s tyres, please consult a caravan tyre specialist.

Words by Simon Barlow, of The Caravan Chronicles

Reproduced with thanks.

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