There have been discussions of tires, viz. purchasing, lately on several forums of Brickboard, and so I thought I would provide some points (some of which might surprise some individuals) here to help in selecting tires.

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First and foremost, all recommended pressures, both by the tire manufacturer for it's max. pressure printed on the sidewall and also by the car manufacturer for the "recommended" pressures (in the owners manual, glovebox, or on the doorjamb), are always all "cold" settings and assume that the tire has been sitting unused for several hours at normal ambient temperatures before the tire is checked for pressure. These manufacturers also know, and decide on pressures to indicate after taking into consideration, that the tire will warm up and their pressures will rise several degrees -- so never adjust them back to the indicated or maximum pressure after just driving them.

Also, do remember that tire pressures change about 1 psi per 10 deg Fahrenheit change in ambient temperature, so your tires' cold pressure will trend toward decreasing in the fall, and increasing in the spring -- more frequent periodic checks of pressure is warranted in these seasons.

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One of areas of confusion that pops up from time to time concerns confusion about car manufacturers' recommended pressure and tire manufacturers' maximum pressure rating on the sidewall.

These are not the same thing. The "max" tire pressure printed on the tire sidewall is the safe maximum limit, as determined by the tire manufacturer. It is not necessarily (and almost always isn’t) the pressure that a car operator should use. Instead, the car manufacturer's recommended pressures are the ones that should be followed -- with some allowance, of course, for driver preference and vehicle use, of course (e.g., more sporty driving and desire to alter handling characteristics, or carrying heavy loads, etc.).

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An interesting example came up recently, involving a Brickboard member who was refused a purchase of tires because the clerk didn't understand the relationship of tire pressure and load rating, and speed rating as well.

To begin, nowadays, it's very, very easy to match tires, even in very different sizes such as a "plus 1" or "plus 2" conversion, to ensure proper load rating for a car. Cars come with original tires that bear a number -- e.g., 87 for a Volvo 240 sedan, or 89 for a Volvo 240 wagon. This number states that the tire is adequate to carry a certain sustained load safely (assuming the operator has inflated the tire to at least the car manufacturer's recommended pressures). Knowing that number, then pick any tire of any other size, whether 14, 15, 16 or 17 inches and in any aspect ratio, and it will be adequate as long as that new tire bears at least the same load rating number (or larger). You know longer have to go to a load-pressure table (as in the "Tire Guide" books) to look this up -- the load rating number, e.g., 87, is imprinted on the tire's sidewall, along with the speed rating (a letter) next to the tire size. As I wrote above, it's now very easy.

So, what about the maximum tire pressures? Doesn't that vary among different tires? Yes, it varies, some having a maximum pressure of 36 psi, some being 44 psi, and others being 51 psi (like my new Michelin Pilots). But (and this is the thing that surprised me when I learned this, and it may surprise some of you, too), these pressures do not necessarily allow one to increase the safe load rating of the tire!

Yes, tire load rating increases with pressure, but only through the range up to (an international, industry standard of) 35 psi*. Increasing pressure beyond 35 psi does not (repeat: not) increase the safe load rating of a tire! I know that a lot of you will question this, and also wonder why, then, are there different maximum pressures -- but this was revealed not only by a TireRack expert but also (after I was still doubtful) a Michelin engineer that I contacted for confirmation! After this, I still had to check further, and found additional confirmation in tables in an industry publication called TireGuide.
( * by the way, this 35 psi cutoff is for all passenger car tires; light truck tires (prefixed “LT”) and “recreational vehicle” tires, e.g., boat trailers (prefixed “ST”), may have a cutoff of 50 or even 65 psi).

As it was explained to me, a passenger car tire with a load rating of 87, for example, can support 1201 lbs at 35 psi. The tire may state on its sidewall, "max load 1201 lbs at max pressure 44 psi", but the tire is actually suitable for that load at 35 psi. And the tire cannot support more load than that at 36 psi, nor at 44 psi, nor at 51 psi -- going over 35 psi doesn't increase a passenger car tire's load rating, whether the tire's maximum pressure is 36 psi or 51 psi; although going below 35 psi will indeed reduce the load rating. Think of this as a curve on a graph, with load rating (Y) increasing with pressure (X), but only until 35 psi where the curve levels off horizontally without further increases in load rating despite increase in pressure.

So, what is the good of having higher maximum pressures for a tire, 51 psi or 44 psi vs only 36 psi, if it doesn't increase load rating? The answer is that it does increase speed rating. We all know that, along with load rating, speed rating is also important. Even though a tire is rated for some high double-digit speed, it isn't necessarily capable of such high speeds without some additional increase in its pressure (and again, this is "cold" pressure). For example, for a V rated tire (e.g., 87V), which is potentially capable of 149 mph, adjustments should be as follows (a 3-column table):
speed, increase in cold pressure (psi), proportion of basic load rating (%)
100, 0, 100
106, 1, 100
112, 2, 100
118, 3, 100
124, 4, 100
130, 5, 100
136, 5, 97
143, 5, 94
149, 5, 91
(this, and the following two tables, are from an industry standard called "ETRTO")

As you can see, this table calls for a limited increase in pressure (over recommended pressure) of 5 psi at maximum speeds -- but never over the maximum pressure of the tire. If we hypothetically propose that the car manufacturer, knowing the weight of its car, calls for a basic pressure of 34 psi, a V-rated tire with a maximum pressure of 36 psi couldn't cope speeds over 112 mph despite the V rating of the tire, since the required increase in inflation from the basic 34 psi to 39 psi (for 149 mph) would exceed this tire's maximum pressure; yet, the tire could be rated for 112 mph, because the tire's maximum pressure would permit an increase to 36 psi, which would allow this speed. However, a different tire with a higher maximum pressure, e.g., 44 psi, can cope with this new pressure.

Also note that at the point where the increasing speeds no longer dictate increases in pressure, at 130+ mph, we then see that the way to prepare the tire for the higher speeds is to reduce its actual load -- i.e., it cannot be rated to carry as high a load as it did at lower speeds -- with the rating dropping as much as 91% of it's original load rating.

The pattern is the same for W tires (rated 168 mph):
118, 0, 100
124, 1.5, 100
130, 3, 100
136, 4.5, 100
143, 6, 100
149, 7.5, 100
155, 7.5, 95
161, 7.5, 90
168, 7.5, 85

and also for Y rated tires (rated 180 mph):
136, 0, 100
143, 1.5, 100
149, 3, 100
155, 4.5, 100
161, 6, 100
168, 7.5, 100
174, 7.5, 95
180, 7.5, 90
186, 7.5, 85

Hope that this helped someone – I’ll add more as they occur to me.

Best regards to all.