INFO

Your Caravan Handbook usually quotes the tyre pressure for the original tyre size fitted and the caravan loaded to the MTPLM (Maximum Technical Permitted Laden Mass). As the load in your caravan can change or on older caravans, the tyre size might have changed 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 wall… this is a typical code:

245/75R 16 120/116 S

The first part of the number is the tread width, in this case 245 mm wide. The second number is the Aspect Ratio which means in this case the tyre wall height is 75% of the width of the tread. The next letter “R” is the construction, in this case Radial, the following number “16” is the rim diameter (in inches). The next number “120/116” 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 250 Kgs to 1900 Kgs. The first thing to check is the tyres are rated for the MTPLM of your caravan, so we need to know the load index ratings.

The other bit of information we need to know from the tyre is the Maximum Tyre Inflation Pressure and Load. This is usually located on the side wall below the tyre data in smaller text. It will say something like “MAXIMUM LOAD 1400 Kg’s MAXIMUM PRESSURE 80 PSI” it might have the pressure given in Bar.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 “120/116” means the maximum load for the tyre is 1400/1250 Kg’s. Remember that is the load for ONE TYRE… so on a single axle those two tyres could carry a maximum load of 2800 SINGLE / 5000 DUAL  Kg’s. 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 2520 SINGLE AND 4500 Kg’s

Once you have the MTPLM 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 MTPLM for a single axle caravan or a quarter of the MTPLM for a twin axle caravan. The resulting figure should be the correct inflation pressure for each tyre.

Working out  tyre pressure for a single axle caravan:

Tyre Details:  245/75R16 120/116 S   – Maximum Inflation Pressure 80 PSI

Weight of caravan: 2000 Kg’s

80 /1400 = 0.0571428 (Max Pressure for tyre divided by load rating)

0.0571428 x (2000 / 2) = 57.1428 PSI (inflation factor times half the weight of the caravan)

So the correct tyre pressure is 57.1428 PSI

Working out tyre pressure for a twin axle caravan:

Tyre Details:  245/75R16 120/116 S   – Maximum Inflation Pressure 80 PSI

Weight of caravan: 3500 Kg’s

80 / 1250 = 0.064 (Max Pressure for tyre divided by load rating)

0.064 x (3500 / 4) = 56 PSI (inflation factor times one-quarter of the weight of the caravan)

So the correct tyre pressure is 56 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”

Speed Category