25% increase in tire pressure during the day (above max)

When a trailer and truck are under tow (by that I mean going down the highway at 55 mph or more) the sun shinning on the tires is basically a non factor.

I just finished a 1500 mile trip with daytime temperature at 95-105°F.

My TPMS never showed more than a 2 psi difference on same axle(cold pressure 80 psi on E rated tires) after at least 20 miles of towing.

I did see pressures in the 95-97psi range on the rear truck tires (cold pressure 80 psi on E rated tire and ambient 100°F). The rear tires on the truck were the ones closest to tire rating on my rig. 2730 load on 3042 rated tires.

When the ambient was in the 65°F range my rear truck tires were running about 90 psi.

Several times one side of the rig was in full sun.

The major factor in tire temperatures while under tow is tire friction from sidewall and tread flexing.

RCMAN46 wrote:
I would consider a 25% rise in pressure close to being a problem........

I agree with this.

If I compensate for the change in ambient temperature - which should result in a 4 psi increase - then the actual pressure buildup is 8 psi - 15%. That is too much!

I think OP needs to use a tire with more load carrying capacity - more inflation pressure (if the tires are rated for such), a higher load range (with an increase in inflation pressure), a larger tire

- OR -

he needs to slow down.

Best guess is that the tires are ST tires and are rated for 65 mph.

The 100 degrees outside tire temp means that the inside is even warmer.

I played with my pressure temp spreadsheet and made picture to make it clear, so first 2 pictures and then the explanation.





You filled 50 psi when outside temp was 70 degr F.
This becomes 61,6 psi when inside tire temp is 165 degr F, when air inside tire is absolutely dry, wich it never is, but will asume it. So when outside 65 degr , inside tire is as I once determined when cold pressure is right for the tire and loading, 112 degr F so 47 degr F higher then outside.
This means for 100 degr outside , inside tire temp normal and save would be 147 degrees.

filled in at the bottom 165 degrees inside tire temp to give the pressure you mention and so about 20 degr F to high.

This can also be courced by severe braking, or sunshine on tire, but then it would be only for the tires in the sun.

If it is courced only by the heat-energy produced by the tire driving at your speed so bending of rubber, it means that your cold pressure is to low and you need higher pressure to bring the warm inside tire temp down, wich is the main goal, and GmandJM already wrote that inside tire temp is more important to monitor then pressure.

If you can produce accurate determined loads on seperate tires, second bes axles, and give maxload and speedcode of tire ( AT pressure of 50 psi=C-load alreasy given) , I can calculate a save pressure for your situation.

Might give that 10 psi higher cold pressure , so 60 psi is needed and most tire-makers allow this.

If that 60 psi is the pressure that gives when filled cold at 70 degrees an inside tire temp of 70+47= 117 degr F only by the heat generated by bending of rubber, wich I state to be ok, the pressure then would rise to 66.6 psi , so about 11% higher
pressure.

The 212 degr F in the middle is boiling point of water , wich some tires inside air can incidentially get by the heat of brakes when severe braking when deschending from the mountains.

Water in tire, can rise the pressure from 70 to 165 degrees by maximum 5 psi, but filled at 70 degr 100% humidity outside air will give no more then 3 psi extra rising of pressure at this temp rising.
So inside tire temp can be lower then my given 165 degrees when expected unintended amount of water in tire.

Totally fine and normal.

Tire pressure goes up about 2% for every 10F rise in temp. If the tires are running or sitting in the sun on one side, the temp. can go up a lot more than the cold inflation pressure. A 25% rise in pressure would suggest a temp. rise of about 100F (above the prevailing ambient temp. when the cold inflation pressure was set), which isn't unreasonable from the sun beating down on the tires after a long trip.

I suggest that you simply measure the pressure after sitting in full shade for at least 3 hours. If not back down to near what you cold inflated them to originally (assuming you did inflate them correctly), then you have an issue. It's good practice to check your tire pressure before heading off each day, especially if the ambient outdoor temp. has changed much.

The max. sidewall pressure indicated on the tires is the max. cold inflation temp. (at prevailing ambient temp) and is NOT the max. the tire pressure should ever reach. The "max" is what you should always set them to - 50 psi for LRC and 65 for LRD.

Tires are designed and tested to withstand about 3 times their max. pressure rating. and are designed to tolerate the pressure increase that happens when travelling at highway speeds. You are nowhere near the danger zone.

Never deflate your tires to adjust the pressure while travelling. Running under-inflated can cause serious internal damage leading to premature failure (blowout) at highway speed. Not good...

For lots and lots of info. on RV tire safety from a tire expert, go to this website

I would consider a 25% rise in pressure close to being a problem.

Pressure Pro has their upper pressure limit set at 24% but can be adjusted between 10-40 % by the user.

Have you ever been on the scales to see if the tires are within their load rating?

What I consider the most important is that all tires on an axle are running the same pressure within in 5 psi after a continuous run of at least 20 miles. This assumes they were at the same pressure when cold and not exposed to a heat source such as the sun.

Just remember the TPMS only transmits pressure about every 5 minutes when things are in normal ranges.

The high temp alarm on my after market TPMS was factory set at 165º F

I have seen my pressures go as high as 67 and the temps up into the 90's on trips temps are more critical to watch, and realize that the readings will vary from front to back and side to side. The important thing is that they are all in the same general range; ie if all temps are say 89-96 they are in the same range but if one happened to be up over 100 I'd stop to see what was up....perhaps a bearing issue. I also always carry an infra red thermometer to double check the TPMS....sort of belt and suspenders.

Pay more attention to your tire TEMPERATURE in that instance.

I have even seen 2 to 4 lbs. difference between the tires on the sunny side of the TT vs. the shady side of the TT.

Tire pressures listed on sidewalls are cold (not driven for at least several hours. even longer when it's hot outside) If you have been driving, the tires heat up even if it's not hot outside, though not as much. Friction causes heat that raises the tire temp. It does this by tread friction on the road surface and by the side wall flexing (friction) that occurs with driving. An under inflated tire will flex more and get too hot leading to catastrophic tire failure.