Tires: tire air pressure and different sizes, tire air pressure, rocket scientists

Question
I have a 99 Miata.  I'm puzzled that when I just bought a set of Doral 195/50 R15 81H M+S tires that the tire store have inflated the tires to around 38 psi.  I am sold old that I use to check the pressure on the wagon wheels(well, not quite).  So I am very familiar with the old bias rayon, nylons and now into the radials.  My manual shows that my tires should read at 28 psi. My tire says the max is 51 psi. One of our larger tire stores put 35 psi on all the tire now matter what size.  When I had my tires bought I waited until the tires were cold in the morning to check out the pressure.  I thought maybe that when they installed that they overfilled so I called the store and he told me that they could be pressure from 40 to 50 psi since that is what the tire says.  Please reply so that I can do the proper pressure before I ruin them.

Answer
Dick,

On every vehicle there is a sticker - commonly called the placard - that will list the original tire size and the proper pressure for that size.  It doesn't matter who made the tire, if the size is the same as the placard, then the placard pressure is appropriate.

I looked up your vehicle in Tire guides - a book that summarizes placards so they are convenient to find - and one of the sizes for a Miata was indeed a 195/50R15 81V, but my book lists the pressure at 26 psi.  That's pretty close - but go by what the placard says!

Now about the store:  They don't employ rocket scientists and the operative word here would be "Simple Instructions".  So most tire shops tell them to inflate the tire based on some simple rule.  Sometimes the rule is "35 psi".  Sometimes the rule is "whatever is on the sidewall."  But rarely is the rule "check the placard", and I suspect there are a number of reasons for that - some of which would involve changing tire sizes.

From these simple rules, the folks that work at the tire shop think that is the "right" answer - which is why I spend a lot of time on the internet trying to point out the placard and why it is there.