Correct vehicle & camper trailer tyre pressures to suit the load carried is one of the essential factors in obtaining good tyre wear & long service life when towing on the highway, not to mention better fuel economy. There are a few ways to work out the tyres correct cold pressure for a specific weight including the 4lb rule, a calculation from the information on the tyre & online calculators. 

Tyre pressures are usually reduced to suit the various terrains such as rough gravel roads, off road trails, sand & mud.

4lb rule from cold pressure

I read about the 4lb tyre pressure rule in Overlander 4wd magazine during the early 80's & have been using it since. It works well for both the vehicle & the camper trailers LT (light truck) tyres, but does not work for low profile tyres.

You need to run at highway speed for at least an hour to give your tyres a chance to build up enough heat. What you are aiming for is an increase of 4lb from your cold pressure reading.

If you are over the 4lb you have not put enough air in the tyres from cold. An under inflated tyre drags on the road generating heat.

If you are under the 4lb you have put too much air in when the tyre was cold. Over inflation when cold will not generate enough heat.

You can build up a chart for cold tyre pressures of varying load weights for both the vehicle & camper trailer.

calculation of cold pressure

single axle maximum tyre PSI pressure	single axle load rating	single axle load rating & maximum pressure

The tyre manufacturers recommended load rating at the maximum cold PSI pressure are found on the tyres sidewall & used in the first part of the calculation to find the correct cold tyre pressure that should be used for a particular loaded weight of a camper trailer or vehicle.

The load rating must first be converted to the tyres maximum kg capacity via a rating / load weight chart. The maximum tyre cold pressure is divided into that kg weight capacity.

Next step the loaded weight of the camper trailer is divided by the number of wheels ie two for the single axle, which is then multiplied by the result of the first calculation.

Our camper trailer LT265/70R16 tyres have a 80 PSI maximum pressure & 121 single axle load rating marked on them. The 121 load rating equals 1450kg tyre capacity via the chart below. 

tyre load rating / tyre load kg weight capacity

example 1 The maximum 80 PSI pressure is divided by 1450 ie 80 / 1450 = 0.05516

The actual 1300kg weight of the camper trailer is divided by the number of wheels for a single axle, then multiplied by the result of the first calc ie 0.05516 x (1300 / 2) = 35.8 PSI

Correct cold tyre pressure for a camper trailer weighing 1300kg is 36 PSI.

example 2 This time the camper trailer weighs 1100kg loaded.
 
80 / 1450 = 0.05516

0.05516 x (1100 / 2) = 30.3 PSI

Correct cold tyre pressure for a camper trailer weighing 1100kg is 30 PSI.