To get the correst tyre pressure there are a few factors involved. One to remember is that it is the air volume and not the pressure that supports the weight, so to get the same amount of air in a small tyre will require more force and therefore pressure than the same amount in a larger tyre. i.e.. 35 psi in a 30"x9.50" tyre may have the same amount of air in it as a 33"x12.50" with 30 psi. I used to have a lot of punctures on my outback trips from stones getting forced into the tyre casing through the sips, (the cuts that are put in the tread blocks). It turns out that having too much pressure opens up the sips and makes it easier to get the stones in. After 7 punctures on that trip and finding that the centre of the tyres had worn I knew that I had to find a better way of getting the right pressures. As it always happens after the fact, I read in a 4x4 mag after I got back that to find the right pressures for different temperatures, loads and conditions you take front and rear pressures cold, and then again immediately after at least 20 minutes at highway speeds or more and there should be a 4 psi increase. If there is more than 4 psi, than you had too little to start with and if there is less than 4 psi, than you had too much. I have used this technic for the last 5 years and didn't suffer one puncture from stones again, (had a steel railway spike go through the trailer tyre on the old Ghan railway line) on the same roads, temperature and loads. The tyres have also lasted longer with very even wear. You can use this technique for your trailer as well. For my tyre size (275x70x16) and load, I run 33 psi in the front and 32 psi in the rear. My mate in his XJ ended up with 35psi front and rear with 30"x9.5"x15 's and later with 33 psi front and rear with 31"x10.5"x15 's. I ended up with less in the rear because I don't carry much in the back of the car, most of it is in my trailer.