The pressure you should run depends on the Load Rating of the tire. Stock is 29 psi. If your aftermarket tire has a higher Load Raing then you can run lower pressure. If your aftermarket tire has lower Load Rating then you should run higher. Ulta low profile tires tend to have ultra low load ratings, so you will need to bump up the presuure.

 

The stock recommended 29 psi is the amount of pressure for the given car's weight for the given tire's load rating.

 

It seams that every time I check the tire presssures after the car has been to the dealer they are wrong. When we bought the car they where set to 35 all around and I corrected it to 29. Since then they usually come back from the dealer around 26-27 and are never the same all around. Right now they vary from just under 26 to 28 after getting the car back from them for bad wear on one tire and an alignment. You would think they would be correct since it was in for tire issues and correct tire pressue and weight in the car are mandatory for a propper alignment as far as I know. After reading many posts in the forum I plan to run 30 from now on.

Why would they ALWAYS lower the pressures in my tires?

 

Even just driving a few miles heats up the air in the tires enough to change the pressure - which is why they should be read BEFORE the car is first driven for the day, NOT after it has been driven to the dealership for service.

I keep my tires accurately inflated to the pressure I want, and so far every time the dealer has had it in the shop, the tires come back to me with assorted too low pressures. Last time I checked pressures just before driving to the dealership - they were right on. When I got the box back, they were all 2-3 pounds low. (...and it was ONLY in for a simple oil change...)

I have instructed the service folks at that dealership several times to NOT touch the tire pressures - to no avail. That is reason number four why I'm looking for a new dealership.

 

Tomas is spot on, as usual.

One of the least appreciated and most underused automotive tools is the tread depth gauge, like this-

http://www.amazon.com/gp/product/B00...X4XG2&n=228013

That's a cheapie, but they work just fine. There are dial iindicator and digital models available as well. Using one every few thousand miles will let you know if the inflation pressure really is correct, and will help diagnose alignment problems early on. Particularly valuable for people running very expensive ultra low profile tires on oversize rims...

Seriously under inflated tires will run hot out on the highway, particularly in the summer, and run a much greater risk of blowout... the problem is exaggerated with heavy loads and the accompanying increase in sidewall flex...

 

While you bring up a good point of tread depth, there is no reason to spend more than a penny for a depth gauge. In fact, just use the penny as a depth gauge.

Place the penny into a tire groove with Lincoln's head upside down. As long as the tread covers part of Lincoln's head you are ok. It means you have more than 2/32" of tread depth. If you do the same thing on the opposite side of the coin and at least part of the memorial is covered you have at least 6/32" of tread depth. If you get below 2/32" it is time to replace the tires.

Also, place the penny in each of the grooves and compare how much of the penny is covered by tread. This is a good indicator of uneven tread wear even before it is visible to the naked eye. Uneven tread wear is an indication of either improperly inflated tires, or a bad alignment.

Find a penny pick it up... all day long you can check the tread depth on your tires.

 

I also like to run my pressures on the higher side, mainly to prevent rim damage in the event of ??? My 215 x 35 x 18's have 38psi in front and 35psi in the rear.