I have one of those utility trailers I haul my ATV on and tires are the small 4x80-12. This morning I was looking up what would be the next size bigger and while I was doing the online chat with the tire rep I asked him about PSI on my tires. The tires on there say max is 90psi but many I talk to say you don't need to. Yesterday I did a trip with it and when I started I had them at 39psi and when I got home the temp and mostly 65mph driving increased them to 43psi. Trailer capacity according to the sticker is 1250lbs and my ATV weighs around 700lbs. The rep said if it says 90, then that's what it should be even if the load isn't heavy. So what is the right way to do it? They fell pretty solid at 40 and I guess I can't get out of my head that putting 85-90 in those little tires wouldn't be right.

 

Max is exactly what that means. It's not the normal running pressure. Where did you purchase the trailer at? They should be able to tell you the normal psi.

 

Bought at local farm store. I have asked them there and they seemed to think not even as high as I usually run them

I run them 40-50 for several reasons which are probably just my own. One is that because my ATV is half the max trailer gross, 45 is half of the 90 max psi. Other reason is I don't have access to a big air compressor any more and the gas station air pumps or the ones you can carry around easy can't pump that much into these little tires. Around 53 is all I can get out of the free air at Casey's (free is why I get air there, why pay 25-50 cents for air?).

The guy I talked to this morning in chat (TireDirect rep) seemed to say that running them low as I do is actually harder on the tires than if I did pump them clear up and that the lower psi could be more prone to a blow out than the high.

 

A lot of tires will show a max psi which is usually for seating the bead during install. It is also pretty common for a trailer tire to have wording on the sidewall such as max load X pounds @ 50psi or possibly max operating pressure = X. On a trailer tire, I always run the max operating psi. Once you put a load on the trailer, the tires will squish a little making the contact patch larger. This generates more friction/rolling resistance and generally more heat. An overloaded tire is more likely to show excessive wear and premature failure.

 

I have 5.3x12 tires and the max load is listed at 80 psi, but if I put more than 30 pounds in them it causes excessive wear on the middle of the tread. I usually run them at 35 psi. I've only had one tire fail and I think it was from running something over.

 

As I said, opinions vary with everyone. This morning I did a search and found a few boating message boards where they discuss the subject (as smaller boat trailers use the same size tires I'm talking about) and from them it seems like the consensus is to run them 2/3 the max pressure listed. So mine says 90, I should be putting in 60.

 

Any time you are running any tire at close to the maximum load you should have the maximum pressure in it. If the sidewalls flex, then they are building heat. A hot tire is a weak tire, especially when you hit a bump. 12" tires are a lot better than the 9" tires but many 12" rims are 5 bolt. The same 5 bolt spacing that you find on 14" trailer, older Chev or Dodge 14 and 15" rims.
If you have room for the bigger rims, then you have ample weight rating to spare and can reduce the pressure to get a softer ride.