tire pressure

[rvp819]

I read all past posts on the subject and googled much but I can't find an answer to my question:

What dictates appropriate tire pressure?

From many members response on this topic, one would assume vehicle weight and expected use would define recommended tire pressure, because most everyone seems to follow Mazda recommendations regardless of tires, but I raise the question because I just had Yokohama ENVigor 15"s installed on my 3 and these suckers are spec'd to a max pressure of 51 psi (!!) compared to 35psi for stock tires (Toyo) and my Canadian Tire winter tires are also rated at 35psi max. Of course, Mazda recommends 32/34 psi, but ...
- what is the car manufacturer recommendation based on? and
- why would someone ever inflate a tire to 51psi (even though it's rated for it)

Your insights are welcome

I just want to make sure I use my new tires correctly. They have very thin/soft shoulders, where the tread meets the sidewall, which I've not seen in a tire before. Would this different structure require higher air pressure?

[notthesnail]

OEM recommendations are based on vehicle weight, tire size, tire load rating, and intended vehicle use. Max psi ratings mean almost nothing unless you're racing or driving on a desert highway in the summer.

Tire pressures should be measured cold. As your tires heat up during use, the pressure will increase, and that's OK. It's supposed to. What you want to look out for is tire wear. Assuming your alignment is correct, you have three basic scenarios:
Too much air pressure - your tires will wear in the centre more than the edges. You will also end up with a harsher ride and possibly reduced traction.
Too little air pressure - your tires will wear on the outsides more than the middle. Your ride will be mushy and traction will absolutely suffer, especially in corners.
The right amount of air - tires will wear evenly across the tread and handling will be more predictable.

You're not likely to see someone inflate tires to 51psi, but 40 or so isn't uncommon for autocross, depending on the car and tires. You might be able to judge whether your tires need more air using a technique that racers use: Chalk or tire crayon (usually white) a few places around the circumference of the outer corner of all your tires. Go from the flat of the outer trear blocks, around the side to the top of the letters. Then go for a typical drive, making sure you corner at your typical speed. Then check the chalk marks to see how far down the sidewalls it's been rubbed off. If it's past the 45 degree mark, add air. *Either do this when they cool, or measure-add-measure so you know how many pounds you added, not what the final pressure is. They heated up, remember?

Here's a scenario:
Measured pressure is 32psi cold.
Tires are chalked.
After the drive, you see that more air is needed.
Measured pressure is now 35psi.
Add air to 37psi.
Tires are rechalked.
After the next drive, chalk marks look good.
New cold pressure is 32 + 2 = 34psi.

Do this for all four tires. Average your results for the fronts separately from the rears, so each axle has the same pressure left and right.

[rvp819]

Very interesting, thanks!