The only way to do this correctly,  Load the trailer and find a public scale.   Weigh the front axle of the truck, record this figure.   Weigh the whole truck, record this figure, then weigh the trailer.          Your tires will have a weight rating on them. Usually on the sidewall, will say someting like  3500# at 60PSI.  This will be the weight rating at maximum PSI of air.    When you do your math on the tires maximum weight rating remember there are two tires holding the load.     There is more to it (to get the correct pressure) but there is limited space to cover everything.          Or the easy way is, take your rig with the axle weights to a good tire shop.   Let them tell you what is right.  

If they are load range "E", 80 psi is the maximum pressure. Load range "D" would be 65 psi, load range "C" is 50 psi..... There may be a tag inside you drivers door near the latching mechanism that would have the correct pressure for the factory tires that were on the truck when new.... If the tire size or load range on the truck is different that whats stated on the tag, then go with the correct pressure thats recomended by the mfg.  

I always run the maximum pressure thats stamped on the tire's sidewall, but that's just me. I'm not one to adjust the air pressure up or down every time I hitch to a trailer. I run 80 psi in all of my load range "E" (10 ply rated) tires.

Here's a handy chart, adjust your pressure according to the load you're carrying.... http://www.goodyear.com/rv/pdf/rv_inflation.pdf

The argument about which tire pressure is correct for hauling a trailer, is as old as Ford vs Chevy, Do I inflate per the door post rating or the tire manufacturer? There are advocates of each in both camps.

The problem arises when you're hauling a load. The door sticker may say to use 55 PSI. This is an average used by the manufacturer to allow a comfortable ride and allow some light loading. The Firestone/Ford debacle was the result of a manufacturer, that was more concerned about ride quality, than the life span of an underinflated tire.

When you are pulling and carrying a load, you want the highest load factor your rig can provide. This is only accomplished when the tires are at their optimum capacity. As was stated, this is marked on the sidewall of each tire, and varies according to the ply rating of the tire. When the tire is at it's maxium pressure, it will provide the most load capacity, have the least rolling resistance and run the coolest. This will ensure the longest, possible longevity.

If you run the truck tires at the door posted pressure, and then put a ton or more of weight on the pin, the tires will become under inflated for the load. In use, the tires will quickly heat up, the sidewalls will absorb most of the heat, at a point the sidewall carcass layers will delaminate, and you will suffer a sidewall blow out.

Idealy it is best to inflate your tires to accommodate the current loading. This can be burdensome if the truck is used for daily transportation during the week, and used to tow on the weekends. Many people will keep their tires at their maxium inflation all the time, including me.

You probably have heard many derogratory statements about this practise. You will suffer a bad ride, wear out the suspension, abnormally wear the center of your tires etc. I even had one person tell me, that the edges of the tire wouldn't touch the payment under a full pressure, light load situation. Unless he installed bicycle tires, this is simply not true.

Like many others, I run the maxium inflation not only on my truck, but my trailers as well. Other than one brand which suffered a catastrophic carcass failure, I have never worn out the centers, or had an overheating failure using maxium pressures. My ride is acceptacle and the truck's suspension remains fully intact.

Another one running the full 80 psi all the time. I also run full pressure in all the trailer tires.

I have not experienced premature tire wear or suspension problems.

 Just be sure that the tires on the truck match the tires on the door sticker...  245-75R-16 lrE.

The pressures listed on the door jamb are not MAX pressures. They are manufacturer recommended pressures. I would not even think about going lower than these. Usually, these are recommended more for ride quality and not load handling capability.

To me, there is not a lot of difference in ride quality from 65-80 psi, however, there is a difference in how the truck handles. More pressure makes for a better handling truck, IMO. I think 45-55 psi is way too low.  

If you weighed the axles or sepeate wheels or wheelpairs, you can take some extra and then fill in the weights in spreadsheet I made, in wich I use the same system as the European tyre-makers use. Then you have to read the maximum load and reference-pressure from the tyres-side-wall and fill them in too( maximum load xxxxkg at yyy kPa ( or LBS and psi). There are more spreadsheets on my skydrive of hotmail , yust search for them , you can navigate my public map the same as in a Forum.

http://cid-a526e0eee092e6dc.office.live.com/browse.aspx/.Public/Caravans%20or%20trailers%20tyrepressure%20advice

in this map spreadsheet for trailers. you have to download it and open it in Excell or likewise programm to use it. directly in the browser wont work because of protection and data-validation I used in it.  And if you cant work it out , give me the details, and I will do it for you.

I found the spread sheet informative. In particular, I entered some common experiences to find answers, of which I was unfamiliar.

A couple of common questions are: If the tire pressures are measured when they are cold, what happens when they heat up? Should the excess pressure be relieved during the day? The first answer of course, is that the tire pressure increases with heat. The second is that no, the pressure should not be released. A third unasked question would include the answer, the tire manufacturers include the higher pressures in their constructions, and the tires are not damaged while containing them.

Using the spread sheet, I entered a starting pressure of 80 PSI on a 65 degree morning, and ended up with a 95 pound pressure when the tires were hot. I was then given the information that the tire's internal temperature was 145 degres. In my usage, these pressures and outside temperatures are what I've commonly measured.

I was pleasantly relieved to find the relatively low internal tire temperature. It is well within a continuous working temperature, that will not damage the tire, and ensure a good long term reliability.

I too, will not use a spread sheet to determine my daily travels, but it can effectively be a useful tool, to decide if what you are doing, is being done correctly.