Wow! Lots to comment on! So allow me to start with how tire manufacturers decide what the max pressure is going to be:
- and for the sake of this discussion, I'm confining the talk to Standard Load Passenger car tires. Other types of tires are similar, but passenger car tires have this odd quirk that confuses things.
First, read up about Load Tables here:
www.barrystiretech.com/loadtables.html
Pay close attention to "The Notes of Page 1-34". Note that the Tire and Rim says that max pressure is either 35, 44 or 51 psi - and no other values are delineated. Why?
Because of speed rating tests. H rated tires are tested at 44 psi (3.0 bar), and V and higher speed rated tires are tested at 51 psi (3.5 bar), while all other tires are tested at 35 (2.4 bar) or 36 psi (2.5 bar) depending on what there max pressure is supposed to be.
Side note: Because the Tire and Rim Association (TRA) is US based, they use English units, while the other tire standardizing organizations use metric units - either bar or KiloPascals. This leads to some quirky situations when talking about pressure. Since P matric tires were first delineated back in the 1970's, TRA uses the round number 35 psi (about 2.4 bar) as the rating pressure for P metric tires, while ETRTO (European Tyre and Rim Association) and JATMA (Japanese Automobile Tire Manufacturers Association) use the round number 2.5 bar or 250 kPa (about 36 psi).
I want to point out that I am rounding the numbers - and that the tests conducted on tires are NOT rounded - but I forget which way they are rounded.
What this all means is that S and T rated tires can have 35, 44 or 51 psi listed as the max pressure, and it is relatively arbitrary which of those 3 is chosen - that is, the tire's construction is NOT specifically designed around that pressure - AND - there aren't any vehicle manufacturers who specify pressures for their vehicles higher than 35 psi (36 psi for metric tires), with some notable exceptions.
Exceptions? German based vehicle manufacturers who specify H or higher speed rated tires, because Germany has the Autobahn with no speed limit, so the vehicle MUST be designed for that and that means the tires have to be capable of handling the top speed of the vehicle - hence the higher specified pressure for THAT condition. However even those vehicles don't specify that higher pressure for normal use.
Note: There are some other interesting exceptions, like the Chevy Astro which specifies 38 psi for a P metric tire, which in theory could be a 35 psi max pressure.
Where all this leads to confusion is that H and higher speed rated tires (The ones that MUST have 44 or 51 psi max pressures!) must also have constructions capable of passing their respective speed rating tests - and many folks interpret that to mean the tire is designed for the higher pressure and that's not accurate. More accurate is that the tire is designed to pass the speed rating test and the construction reflects that.
HOWEVER, the DOT endurance test is conducted at 35 psi (or 36 psi for hard metric tires) regardless of what the speed rating is - hence the construction changes for higher speed rating tires are only their for the speed.
So I call the pressure listed on the vehicle tire placard as a "Specification" - sort of like the oil viscosity is specified for the engine. You can do something different, but you have to be aware that the vehicle manufacturer doesn't endorse it.
