I have mickey thompson mtz's 315.70.17 on my 2015. I have them set to 30 psi now and seems better than the 37 psi on the door placard. After driving and getting the temps up on the tires they show 32 to 33 psi on a 85* outside temp day. My question is what is a normal increase in psi once warmed up and what is an indicator of to much or to little psi.

 

35" tires should be 27-28 psi generally for the best even wear... It is normal for tire pressures to increase with temp.

 

OK so a 3-5 psi increase isn't a sign of under pressure? I know about the chalk test I'm more interested in under inflation causing high psi

 

From my motorcycle background I use the 10% rule. Looks like are good i.e. From cold psi to fully warmed up psi should raise 10% less than 10% too high more than 10% to low. So if you are 30 cold and 33 hot that's perfect 10% of 30 is 3. Been using that method for about 30 years lol.

 

The delta is (typically) 1psi per 10°.

 

Outstanding thanx for the comments

 

With all due respect to the members of this forum--and we have some really sharp ones!--I'd say trust the engineers who designed the tires on this one. You can go through the complex calculations they used, or just use the chalk test with cold tires. After that, don't worry about it. The engineers took Charles' Law into consideration when they designed your tires and set the limits on pressure.

 

The concern isn't the increase in pressure but too much of an increase in temperature from flexing when pressure starts out too low. The heat will cause the tire to break down.

 

The chalk test for me has shown fine from about 10psi to 40psi and the jeep engineers have not determined what my 35" e rated tires should be run at and toyo engineers tell me what they need to be run at when carrying max load. I'm somewhere under that . 10% rule has always worked my tires wear incredibly even and my ride and handling are great. I agree tho if I was running stock oem tires I'd do what is on the door sticker.

 

Charle's law is an isobaric situation. While the workings inside a tire are anything but stable the proper technique would be to use the isochoric situation.