TREADWEAR
The treadwear grade is a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specified government test course. For example, a tire graded 150 would perform one and a half times as well on the government course as a tire graded 100. The relative performance of tires depends upon the actual conditions of their use, however, and may depart significantly from the norm due to a person's driving habits, maintenance, and differences in roads as well as climate.
TRACTION
The traction grades from highest to lowest are AA, A, B, and C, and they represent the tire's ability to stop on wet pavement as measured under controlled conditions on specified government test surfaces of asphalt and concrete. A tire marked "C" may have poor traction performance. The traction grade assigned to tires is based on straightline braking traction tests and does not include cornering traction. In a straightline stop, the "AA" graded tire should allow you to stop your car on wet roads in a shorter distance than tires graded "A," "B," or "C," and obviously tires graded "C" offer the least traction on wet roads.
TEMPERATURE
The temperature grades are A (the highest), B, and C, representing the tire's resistance to the generation of heat and its ability to dissipate heat when tested under controlled conditions on a specified indoor laboratory test wheel. Grade C corresponds to a level of performance that all passenger car tires must meet under the Federal Motor Vehicle Safety Standard No.109. Grades A and B represent higher levels of performance on the laboratory test wheel than the minimum required by law and are cooler running tires than those rated Grade C. Sustained high temperatures can cause the tire material to degenerate, and excessive temperatures, underinflation, and overloading can lead to blowouts and tread separations. If the vehicle and tires are maintained and used properly, they should perform safely and reliably when run at the speed limit, even in very hot weather.
Here are Rob's bias-ply 5.90x15...
Here are Rob's bias-ply 5.90x15 Firestone and B.F.G. Silvertown tires in a whitewall on his '53 Chevy from Coker Tire. Radials can't hold a candle to the classic looks of a bias-ply tire, which is why they still have a legion of followers after all these years despite the fact that radials offer better gas mileage and all-around performance. For those who want to compromise between the two, Coker does offer whitewall and redline radials.
SERVICE TYPE & SIZING
A tire's size is usually preceded with a letter designation that shows what that tire is intended to be used for. The two most common service types are "P" and "LT" and are seen on a tire's side, like this on the tires on my '72 C-10 P225/75R15 or LT225/75R15. Most radial tire sizes are given in a metric measurement, and the "P" is the metric designation for passenger car and light truck tires. "LT" is also a light truck tire, but is usually given a higher load index rating. A "T" prefix means the tire is for temporary use as a spare and shouldn't be used for regular driving. There are other service type ratings, but most are for super heavy-duty applications that most of us will never have to know.
The first three digits of a radial tire's size are the section width given in millimeters (mm), which is the distance between the widest points of the inside to the outside of the tire's sidewalls when mounted on the proper wheel and inflated to the required psi. To convert this into an inch measurement, we divide the section width in mm by 25.4 (mm per inch), so the same P225/75R15 tire has a section width of 225mm, or roughly 8.88 inches.