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. 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 all caravans.

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:

**205 / 65 R 15 94 H**

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

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 670 Kg’s. Remember that is the load for ONE TYRE… so on a single axle those two tyres could carry a maximum load of 1340 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 1251 Kg’s

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 720 Kg’s MAXIMUM PRESSURE 52 PSI” it might have the pressure given in Bar.

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.

**Lets 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: **1200 Kg’s**

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: **1800 Kg’s**

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, your caravan manufacturer or local dealership will be able to advise you fully. If you have any doubts about the suitability or the condition of your caravan’s tyres please consult a caravan tyre specialist. Remember any tyre defects that you can be prosecuted for on your towing vehicle, also apply to your caravan tyres when towing your caravan on public roads.*

Ian

said:Simon, this is an excellent page of advice regarding tyre pressures. I think caravan owners are being given poor advice regarding cold tyre pressures in their Handbooks and tables on the web. Even the TyreSafe website only gives maximum designed pressures for different tyre sizes and load ratings. No advice is given for lower MTPLM’s. I have written to TyreSafe expressing my concerns and they haven’t replied. I am convinced that because of this poor advice many owners are towing caravans with over inflated tyres.

Robert

said:Hello Simon

Yet another great piece of information published on Caravan Chronicles for which I thank you.

After reading this blog I checked out the tyres on my van, mainly to access what the correct pressure should be on my caravan. To my horror I found that the tyres fitted were way below the advised load rating for a caravan with a MTPLM of 1400kg. The one thing in their favour was that according to the date mark they were inside the five year life, but were three years four months old. The tyres on the axles did not match the handbook in size as they were 165/14R and not 175/14R as stated.

When I bought the caravan from a local dealer I used it solely as a base at my Gliding Club and for the one trip a year to the regional competition. However things have changed and now my partner and I are touring and safety is a major priority. New tyres have been fitted which leave a margin over the 90% of load rating, and Tyron bands have also been fitted for added safety on motorways.

Oh yes, and the tyre pressures are spot on thanks to your formula, as is the torque rating which I will re-check after a short circuit of a local dual carriageway.

Many thanks

Robert

Steve

said:Extremly Helpfull

david

said:great piece of advice Thank you

Graham Tomlin

said:This article has given me the best answer to date as all others just specify the max PSI. I have been quoted 58 PSI but using your formula I make it at 44 PSI. Does the calculation need to take into account whether the wheels are alloy or not please?

The Van is a single axle with a 1200KG MTPLM and the tyres are:

‘185 R14 C 104 / 102 N Load single 900Kg at 450KPA (65psi) cold’

GT

Simon Barlow

said:Hi Graham

It doesn’t make any difference if the rims are alloy or steel.

The only thing you need to keep an eye on is the pressure when travelling to hotter or colder areas. For example if you set the pressure in the UK in winter then headed down to Spain for a month over winter, you will need to check and adjust your tyre pressures again so they are the same for the warmer climate. It’s something we don’t think about so much but in the USA the RV’ers and travel trailer haulers are aware of this much more than we are.

S

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Gary Hepworth

said:I recently had the tires changed on my 2009 Lunar Clubman ES Caravan. The manufacturers handbook states running at 73psi cold. I have always thought this to be too high. The old tires showed signs of wearing in the center of the tread, a sure sign of over inflation. I queried the pressure with Lunar ( I have done this on at least two other occasions) and this time they admitted it was a mistake. Advice was to go with the new tire manufacturers advice at 65 psi. The tires are GT Tires Kargo Max ST600. I am going to do this for the time being, but I am going to spend time and calculate the pressure as I believe it should be a little lower still!

Simon Barlow

said:Hi Gary

This is not the first time someone has reported to me (via the blog) that the original pressures given by the manufacturer (not all Lunar by the way) were too high.

Gary Hepworth

said:Hi Simon. This is our first caravan, new in 2009, so we took all info from the manufacturer as set in stone, so to speak. The tire wear is what really done it for us, although still legal the signs were obvious. Some of my friends are towing vans with the same weight and running at as low as 55 psi with no problems. It’s the safety factor that concerns me most. This was not helped by Lunar’s almost cavalier attitude to what could have been a potential disaster for myself and other road users. The van was skittish on a couple of occasions, but we put this down mainly to road condition and passing HGV’s etc. I now think different. We do have Tyron bands fitted (and now we carry the TYRON fitting machine) but I’m hoping never to test them. If your site manages to save other caravan users from experiencing something pretty horrific, then it really will have done a good job.

Kind regards, Gary Hepworth.

Ken Lord

said:Thank you for providing a very informative & easy to follow calculator for tyre pressures. Unless I have missed something…is the formula based on the aggregate weight of the van or, is it the gross ie taking the ball weight into account. Kind regards Ken Lord

Simon Barlow

said:Hi Ken

The formula is based on the axle weight, so deducting the nose weight from the total weight of the caravan is the correct way to go.

Alko chassis caravans have a max nose weight limit on them of 100Kg, if you do the maths 100Kg difference is only a tiny PSI difference so personally in practice I just use the weighed weight of the caravan to work out my tyre pressure.

Being slightly over to my way of thinking is better….air leaks out, I have never heard of it leaking in.

Barry Russell

said:The ball weight is not carried on the van tyres so I assume it would not be included.

Ken Lord

said:Barry,I’m not sure about your comment. My van has an aggregate weight of 1600 kg of which 160kg is ball weight, resulting in a gross weight is 1440kg…..in effect meaning there is less weight on the ball. Thank you for replying. Cheers Ken

Simon Barlow

said:Hi Ken,

160Kg seems high for the nose weight…. is it a Winterhoff/BPW chassis and hitch?

Ken Lord

said:Thank you Barry & Simon…I do appreciate your thoughts on this subject. My van a 17′ (external length) Galaxy pop top is very traditional in construction….Preston? built duragal chassis, with conventional A frame & coupling. It is reasonably heavy at 1280 kg Tare. Perhaps with some of the new Euro & South African built vans, they may be designed to have a lower ball weight. However from all that I have read, the recommendation is that a van should have a minimum of 10% “on the ball.” Barry, I like your comment re “air not leaking in”…ha ha.. So for the future I will adopt the aggregate weight to calculate tyre pressures. Cheers Ken

Anthony

said:Hi there, does the same principle apply to vehicles, eg. cars, vans with non original wheel and tyres?

Thanks

Simon Barlow

said:Hi Anthony

Yes and no.

Let me clarify that a bit. 99% of the time yes it works, however there are a few exceptions for mainly high performance and 4 x 4 vehicles.

High performance vehicles can have differing pressures from side to side as well as from to rear. In this case stick to what the vehicle handbook states.

For 4 x 4 vehicles, some run what may seem as slightly odd pressures. This is to ensure that the rolling diameter of all four wheels is exactly the same even though there are axle loading differences to minimise torque wind-up of the differentials.

Additionally there are a few tyre types that don’t obey the rules…. biased ply tyres are one example where pressure dictates how the tyre launches from a standing start.

Anthony

said:Many thanks, this is a great thread and I’ll be using the calculation to work out roughly the correct inflation for the non original wheels on my van, using weight info for front and rear axle in the handbook. I can at least try and see how they wear I suppose?

Simon Barlow

said:Work out what pressure it should be with the tyres you have and double check what the original tyres were. If its close you’re good to go. If its big difference then see if you can find a similar weight vehicle with the same tyres and find out what pressure they run at.

Another check would be to monitor the tyre temps… inflate to your calculation and do a modest run for 30 min and check the temps and adjust as required.

Jadatis

said:Been following this topic since the beginning.

As” pigheaded Dutch selfdeclared tyrepressure- specialist” made some spreadsheets for pressure advice for motorhomes and caravans.

Placed them on public map of one-drive beonging to my hotmail.com adress with username jadatis.

Will see if I can place link to it on mobile phone

Much about tyrepressure and seach the map you need.

https://onedrive.live.com/?gologin=1

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Peter

said:The 205/65R 15 94H is a standard load normal car tyre wich carries it maxload of 670 kg up to 160kmph/99mph AT 2.5 bar/ 36psi.

The difference between 36 and 60 psi is used for higher speed and camber angle above 2 degrees .

A reinforced / extraload/ XL i n same sise would have loadindex 99/ 775kg AT 2.9bar/42 psi.

So for caravan use you can calculate with 36 psi. Further the lineair calculation you do is not that bad, gives slightly higher pressure then official calc.

But first add 10% to Load before calculating, then you get highest reserve with still acceptable ” comfort” and gripp, so still no screws trembling loose.