I decided after writing the review of my Bridgestone S-03s that I would post information about the different size numbers and specifications found on the sidewall of your common passenger car tire. Many details about your tire can be found in those codes such as size, load & speed ratings, durability, and performance.

TIRE SIZE
New passenger car tires are sized with a 3-part number.
For example, My330i's front tire is a 225/40YR18.

225	WIDTH
40	ASPECT RATIO
YR	SPEED RATING/TYPE
18	WHEEL DIAMETER

WIDTH
A tire's width is measured in millimeters (mm) at the widest point from sidewall to sidewall.

ASPECT RATIO
Height of the sidewall. It is measured as a percentage of the tire width, so an aspect ratio of 40 on my330i tire is 40% of the width or 90mm.

TYPE
Typically, first letter is the speed rating (explained below) and then R for Radial construction.

WHEEL DIAMETER
The diameter of wheel used for this tire in inches.

SERVICE DESIGNATION
Right after the tire size* marking on the tire is the "Service Description" or load/speed rating.
My330i's front tire is a 88Y SL, while my rears are 91Y SL.

The number part of the Service Description designates the weight the tire can support at max inflation (also printed on the sidewall, in my case, 51psi). Each load number and the weight they support is listed below:

Code	Lbs. (Pounds)		Code	Lbs. (Pounds)
71	761		91	1356
72	783		92	1389
73	805		93	1433
74	827		94	1477
75	853		95	1521
76	882		96	1565
77	908		97	1609
78	937		98	1653
79	963		99	1709
80	992		100	1764
81	1019		101	1819
82	1047		102	1874
83	1074		103	1929
84	1102		104	1984
85	1135		105	2039
86	1168		106	2094
87	1201		107	2149
88	1235		108	2205
89	1279		109	2271
90	1323		110	2337

So, in this example, my front tires (88Y) can support 1,235 pounds at maximum inflation pressure (51psi), while my rear tires (91Y) can support 1,356 pounds.

The LETTER part of the Service Description designates the maximum speed the tire is rated to operate safely at. The rating is not supported if the tire is damaged, punctured, repaired, underinflated, overloaded, or abused. The manufacturers would never recommend exceeding speed limits anyway.

Code	MPH		Code	MPH
N	87		U	124
P	93		H	130
Q	99		V	149
R	106		Z	149
S	112		W	168
T	118		Y	186

* Z-rated tires don't carry a Service Designation since the Z is embedded in the tire size such as 225/50ZR16.

UTQG RATING
Elsewhere on the sidewall, you'll find the governmental Uniform Tire Quality Grade information.
My330i's tires are "220 Treadwear AA traction A Temperature".

TREADWEAR
Treadwear ratings are conducted by the tire's manufacturer and are really only good at comparing tires from the same manufacturer. The idea is that a 200 treadwear rated tire will last twice as long as a 100 rated tire. I find it very subjective as my 220 rated Bridgestones on my BMW lasted just as long as 320 rated Michelins on my Maxima.

TRACTION
Ranging from AA, A, B, or C, the traction number is based on straight-line BRAKING performance in wet conditions. AA being the best. It DOES NOT indicate cornering ability.

TEMPERATURE
Ranging from A, B, or C, the temperature number is based on the tire's resistance to heat under high-speed driving conditions. All tires must meet a C rating, but of course, an A is the best. Winter and light truck tires don't normally carry a temperature rating.

CORRECT TIRE PRESSURE
The most important thing you can do to keep your car and it's tires in top condition is to regularly check your tire pressure. Incorrect tire pressures cause wear, bad gas mileage, improper handling, and can contribute to the premature failure of a tire possibly resulting in losing control and a collision. Every tire has it's maximum pressure rating on the sidewall usually stated as "MAX LOAD #### LBS AT ## MAX PSI". It is very likely that you will never run at these pressures. Each vehicle manufacturer states the recommended tire pressures on a placard on the driver's door frame (as in our BMWs), glovebox, or fuel door. This is a good place to start.

If you have changed your tires, say going from a 17" to an 18" wheel, add (2) TWO psi to each tire pressure for each inch of wheel increase.

I will use my330i as an example: BMW recommends that I run 30-35psi front and 35-42psi rear on the stock tires depending on how much cargo/passenger weight is onboard. Since I have upgraded to 18" (or +1 wheel size), I started by adding 2psi to each empty-laden pressure resulting in 32/37psi front/rear. This is a safe place to start, but adjustments in handling can be made by adjusting air pressures. BMW builds understeer into our cars by design and having larger sized tires (sport pacakge) in the rear just adds to the understeer condition. Now, understeer is great for safety, but no fun in the turns for an experienced driver. I simply brought my tire pressures to 35-36 all around it really helped reduce understeer.

I would never go below the recommended tire pressures on our cars. An overloaded tire will give you no warning when it lets go. Make sure to check your tire pressures at least twice a month and always check tire pressures when they are cold (haven't been driven on in a few hours).

A good rule of thumb: If you can see the tire bulging at the sidewall, the air pressure is already too low.

In addition to the primer here, you can find a great wealth of information about wheels, tires, and suspension components and their operation at TIRERACK's Wheel and Tire Tech sections.

PLEASE NOTE:
This information is based on factual data I have researched and if you feel any of it is incorrect, inaccurate, or missing, I do apologize.