If I run the OEM tires for my car/truck, I should run at the recommended pressure. OK I am fine with that. However, if I change to a different size/style of tire what guide should I follow, the one on the sidewall of the new tire or should I stick with the manufacturer recommendation? This is specifcally related to my wifes Volvo not my truck.

 

the psi on the sidewall of the tire

 

Even with my OEM tires, I run the pressure on the sidewall.
I figure the people that made the tire know more about it than the people that chose to put it on the vehicle. Besides, after the fiasco ford created, Im not taking a chance on under inflated tires just so I cant feel the road.

 

The salesmen selling you the tires is your guide.

That way if that tire pressure gives you poor treadwear, you got someone to pick on.

The tire manufacturer has no idea what rim and what weight of vehicle that tire is going onto. Forget anything written on the sidewall.

 

The rating on the sidewall is for MAX load.
Running your truck empty with the tires inflated to the max pressure might save you a buck or two in the wallet for fuel but will cost you big bucks in premature center of the tread tire wear.

Use the chart in your owner's manual, or the label on the driver's door for a good starting point.

Case in point:
My truck,,,sidewalls say 80psi, I run them at 55psi because the overinflation wore the center of the tread out when inflated to only 70psi.

 

So running 22 psi in a Ford Excursion was a good idea/??

 

My owners manual has a chart that shows the proper tire pressure based on the weight you are carrying. I have the stock Michelins and run them at 50 psi when empty. They currently have about 30% left with over 40K miles on them.

 

Exactly as Shovelhead and Timmay said, tire press on sidewall is MAX psi at max load. Yes, you can put this in it as that is why it is printed there, to tell you the maximum amount of safe pressure at rated load. The sticker on the door is not correct either when you go to changing the tire size. It is however a starting point. Think of it this way however, the larger the total surface area of the tire the less psi, which is of course pounds per square inch, the less psi is needed to support the vehicle at a constant weight. Think about the psi this way, my 39.5" tall 15" wide tires will still support the wieght of the truck at 2psi! Since you are talking about a car though I wouldn't vary much from the door because there isn't enough room to fit a largely different tire. Start with the door and if you notice wear in the center, decrease pressure and if the wear occurrs on the outside of the tread, increase pressure.

 

If I put a different type of tire on I start with the book, but... I take the truck to a place that has level pavement, use a large builders chalk to put several wide lines of chalk across the tread surface then drive on the pavement for about 30 yards or so and check the marks. Worn off the sides,,underinflated, worn in center,, over inflated. Keep adjusting until even wear is shown and make a note of pressures then adjust from there for added weight. Takes time but it works.