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…
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. Just as important is ensuring those caravan tyres are inflated to the correct pressure.
The simple fact is, caravan tyres often get overlooked and are not checked regularly. 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 caravan tyre pressure.
Your caravan handbook usually states 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 you or the previous owner may have changed tyre size), it’s worth knowing how to calculate the correct pressure from the markings on the tyres fitted to your caravan.
You first need to decipher the code on the tyre sidewall. This is what a typical code looks like:
205 / 65 R 15 94 H
The first part of the number refers to the tread width, which in this example is 205mm wide. The second number is the Aspect Ratio which means the tyre wall height is 65% of the width of the tread.
The next letter – R – is the construction, which in this case, is 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.
Caravan tyre pressure is confusing, right? Bear with me though.
The load rating for the tyre can range from 60 to 130, equating to a maximum load per tyre of 250kg to 1900kg. The first thing to check is that the tyres are rated for the gross trailer mass (GTM) of your caravan. As a result, we need to know the load index ratings.
From the table above, you can cross-reference the load rating code number to the 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 bits of information we need to know from your caravan 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 will 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.
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
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
As a rule of thumb, the inflation pressure of a hot caravan 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, this means the cold inflation pressure was too low. If the hot inflation pressure is less than 4 PSI difference from the cold pressure, then the cold pressure was too high.
This is because 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.
1.You give not to use more then 90% of maximum load of tyre, but calculate pressure for 100% of gvwr.
Luckyly in australia mostly 10% on towbar, so 90% on the wheels, so it becomes automatically 90%.
In Europe minimum 1% on towbar, so then to low reserve build in.
2. You give 60 psi maximum cold pressure for the tyre in the example. But the maxloadpressure (for wich the maxload is calculated for 160kmph/99mph) is if standard load normal cartyre 36 psi, and if XL /reinforced/extraload 42psi. Then this gives lower outcome.
On LT (lighttruck) and C(comercial), and trucktyres they only give maxloadpressure on sidewall, on normal cartyres only maximum allowed cold pressure, between 44 and 51 psi, and an ocacional XL// 60 psi.
Hi Guys,
The info on tyres is excellent and great to know. Handy to print out and laminate and keep in a folder for future reference or camp fire arguments. LOL.
Another great story – if not published before- is headlight adjustment, what happens to your low beam when you add even 50kgs to the back of the car/suv/4wd. Most cars these days have LED lights and are brighter than the old halogen style, coupled with now non adjustable from cab (????), which even most people didn’t know how to use. Correct adjustment is even more imperative as people change their old style to new style of lights or even after a minor accident. Repairers don’t adjust lights, and it can as blinding as a goose with full high beam on. Especially trade utes with 500kg of tool boxes and beer and camp gear. Just an idea to help keep it fun.
Love all the articles in general.
Ian
I just wish more people would read stuff like this and not drive with underinflated tyres. My caravan plate says 350kPa, should read 450. The van came with tyres too small for the load. I had to go from 195R14C to 205, and thinking of using 215 for a bit of safety margin. So from 950kg (for a 2t van) to 1120kg. Needless to say I am very mindful of my tyre pressure and age.
The biggest problem are the service stations. Unless they have a truck rated compressed air facility they will max out at 45PSI, just over 300kPa. Well below of what most single axle caravans require.
Even the ARB compressor I had to buy to pump up my caravan tyres on the road did that. Supposed to be rated at 100PSI. After a big argument they did replace a pressure switch with one that is 100PSI rated, not the cheap one they install and hope no-one will complain.
Correct tyre pressure and tyres well past the service life are a big problem, especially for single axle. There is zero redundancy and plain dangerous. We are lucky tyres are made to be abused but some are pushing that luck somewhat endangering lives of others.
Hi, does this take into consideration any weight that is/will be carried by the car when the caravan is connected or is that not a factor?