How did I miss this discussion? Oh, well. It seems the party is winding down, but there's still some dip left......
What I am about to write is backed up with more detail here:
http://www.barrystiretech.com/loadtables.html
It is a common misconception that the inflation pressure listed on the vehicle tire placard applies only to the OEM tires. No. It applies to ANY tire of the same size. This is all about load carrying capacity.
Will those replacement tires perform differently than the OE tires? Yes, of course. Different tires can be designed to perform differently, and because the OE tires were tuned to the likes of the vehicle engineers, they therefore reflect what ride and handling characteristics THEY wanted. The difference is the difference between who did the ride evaluation and what was desired and not a difference in the load carrying characteristics.
So it's important to know what tires you are purchasing do, if ride and handling are a characteristics of importance to you.
Will changing the inflation pressure change the ride and handling characteristics? Yes, but changing the inflation pressure also changes the load carrying capacity and that could profound effect on other things.
What other things? Adding inflation pressure reduces the size of the footprint and since grip is related to the amount of rubber in contact with the ground, it reduces the grip. (and , yeah, the relationship is quite complex)
Adding inflation pressure increases the spring rate of the tire, and while that causes the tire to respond to steering input faster, it also makes the tire more prone to impact damage.
There's another question about the differences between the load carrying capacity of tires. They aren't the same.
This is an artifact of the difference in the way the Tire Standardizing Organizations have described the load carrying capacity vs inflation. It's not that the tires are behaving differently, it's that these organizations took different paths in their theoretical studies. If you look closely, you'll see there is little difference between them.
Back in the day, they used pencil and paper (and a sliderule) to figure these things out. There is only a certain amount of precision you can get with those tools.
Today, we'd use high speed computers and get a much more accurate answer more quickly. Unfortunately, once the load tables have been published, they can't be withdrawn. They are kind of "cast in stone".
Good news! A couple of years ago, all the tire standardizing organizations agreed to use the same formula, so there won't be any differences on newly introduced sizes. However, the old sizes still carry over. If a new sizing system is introduced, it will be the same worldwide.
Now about about what is stamped on the sidewall of passenger car tires?
There is a regulation that is worded rather peculiarly and the result comes out one of 2 ways:
1) Max Load XXXXX at YY pressure
This describes a relationship. Notice this doesn't state what the maximum inflation pressure is.
2) Max Load XXXXX, Max pressure YY.
These are 2 independent statements. Notice it doesn't state a relationship.
For standard load passenger car tires, the max load occurs at 35 psi (36 psi for those expressed in metric units). The maximum inflation pressure could be 35, 44, or 51 psi - all with the same max load. The other inflation pressures are there for speed ratings.
So any particular passenger car tire can be stamped a number of different ways and still be correct. In all cases, there is a load table that states what the relationship is and taking into account that the tables are slightly different due to the standards organizations, they are all the same.
So for passenger car tires, the relationship would be stated:
Max Load XXXX at 35 psi (or 36 psi for metric units)
- OR -
Max Load XXXX, Max pressure 35 (or 36, 44, 51) psi.
Occasionally you will see some tire manufacturer make a mistake and state it this way:
Max Load XXXX at 44 psi (or 51 psi) - and I think this isn't quite accurate. It isn't exactly wrong, but it implies something that isn't there.
And one last thought: Inflation pressures are all stated as "cold" - meaning at ambient conditions - except where it is clear they are talking about pressure buildup. The maximum pressure listed on the sidewall is a "cold" pressure and the tire is designed to withstand the higher pressures due to the build up cause by heat. You can safely exceed the maximum pressure when operating a tire, if you started out below the maximum value.
