Tires: Tire pressure, good year tires, inflation pressure

Question
Please give me the correct pressure for Rv trailer tires weight on the two axels is 6980lbs tires are good year marathon 225-75 R 15 load range D max load 2550 @ 65psi The original tires were load star st225-75-D 15  max load 2550 @ 65psi info on trailer says inflate to 65psi and so does the dealer, trailer mfg and tire mfg..But the tires wore on the inside and outside treads and the tire mfg said this was low air pressure. The tires never had less than 65psi in them when on the road. I used 3 tire gauges to make sure the gauge was correct. I think the tires were over inflated. I want to have the correct pressure in the Good Year tires.   Thanks Robert

Answer
Robert,

The maximum load carrying capacity written on the sidewall of a tires is for ideal conditions.  You should always inflate for more than that.  

For trailers, I recommend 15% more calculated load carrying capacity than the actual maximum load.  So let's see how that works out.

The trailer weighs 6970#.  If I deduct the tongue weight (which is supported by the tow vehicle) of 10%, then the tires have to support 6273#.  115% of that is 7213#

4 tires @2550# each = 10,200#

That is less than the minimum needed, so if I use the load vs pressure tables for an ST225/75R15, I get that the inflation pressure has to be at least 40 psi.

But there are parts of what you said that do not make sense.  Usually excessive wear on both shoulders indicates underinflation.  However, it is extremely unusual for trailer tire to wear out.

Most tire wear occurs in the cornering mode - and most trailers spend a lot of time being pulled on long trips so they hardly ever wear out.  Usually age catches up with them first.

So how sure are you that the entire rig weighs 6970# fully loaded - especially fully loaded the way YOU load it, not the way the manufacturer thinks it ought to be loaded?  

My experience says that trailer manufacturers do not do a good job on sizing tires.  They typically underestimate the amount of stuff people put in their trailers, but the size the tires they spec are barely adequate for their underestimates.

Your numbers don't seem to reflect that.

Based on what I know - and based on the way the tires wore, I think 65 psi is right inflation pressure.  And certainly, over inflation would cause the tires to wear in the center of the tread - the opposite of your experience.

But here's a test you can perform to help you decide if the inflation pressure is adequate.

The next time you use your trailer, check - and record - all the inflation pressures.

Then go on your trip, but stop after an hour and immediately check the inflation pressure.

The buildup of pressure for any particular tire should not be more than 10% - in your case, 71 psi.  If it is larger than that, especially if it is more than 15% (75psi), then you need to increase the load carrying capacity of the tires.  Normally this means using a higher inflation pressure, but if you've maxed out the pressure, you need a larger tire.

Pressure build up less than 10% is fine - particularly if it is couple of psi.  That means the tire is not overheating.