Have you ever gone into a tire store for new tires and had the salesmen ask what size you need? Chances are, you didn't know, so he went out, looked at your tires. Then he came back saying something that sounded like he was reading alphabet soup: "You need a set of HR-250/70 by fifteens." Then he wrote up the bill, you signed it and went on your way, somewhat poorer and quite confused. Believe it or not, tire jargon, like any other "technical talk" does have its meanings; it makes sense, once you understand the code.
To understand this, let's look at the example above: P250/70-HR15.
We'll start with the numbers, first. The big one -- 250 -- is the tire's overall width, in millimeters. It's what you'd get if you measured the width from the outside edge of one sidewall to the other. The bigger this numer is, the wider your tire will be.
Next, the "/70." That's the tire's profile height -- what you'd get when you measure from the ground to the edge of the wheel rim, divided by the width of the tire. Let's say you have a tire with a profile that is 175 mm high and a width of 250 mm. Dividing 175/250 = 0.7; meaning the sidewall is 70 percent as high as the tire is wide. Note that the lower the number, the lower the sidewall height is compared with the width. If you have a 200mm width tire, a 70-profile would be 140 mm high, a 60-profile would be 120mm high and so on down to a 35-profile, which would be only 70mm (2.8 inches) high. That is very low profile, indeed.
The "x 15" is the weel rim diameter, measured in inches. This mixture of metric and standard/English units may be a little confusing, but that is what happened when foreign tire companies started making tires for American cars. Back in the 1960s and before, tire width was denoted by a letter -- F, G, H, J, etc. each one stood for the width of the tire in inches, with the higher letter meaning a wider tire. The system has been replaced by the less confusing and more direct metric one.
The letters need a bit more explaining. "R", as you might guess, stands for "radial." That is, the reenforcing fabric is wrapped around the carcas (the whole tire, from bead to bead) directly across the tire, as if you drew lines from the center to the tread. A "bias ply" tire has its fabric reenforcements placed diagonally, as if you were wrapping a bandage around it. Because radials have lower rolling resistance, you're not likely to find them available for passenger cars.
The letter "P" designates a tire intended for use on Passenger Cars. There are other designations for Temporary-use spare ("T") and Light Truck (LT), as on pickups and SUVs.
So what's the "H?" That stands for the speed rating. Some tires meant for low-speed use (around town, etc,) don't come with this, but tires intended for high speed use are rated according to how fast the tire can go for extended periods without fallng apart from centrifugal force and road contact. These letters are:
S -- up to 112 mph
T -- up to 118 mph
H -- up to 130 mph
V -- up to 130 to 149 mph (depending on service ratings)
Z -- over 149 mph.
These are tire industry standards, so you can use them for reference, but don't count on using the full speed capacity of the tire unless you like talking to police officers, judges and court clerks who collect fines.
Three other ratings you might find on a tire are: Temperature, Traction and Tread Ware. The first two are graded A, B, C (where A is the top rating), and tread ware as a number, usually 80 to 150, give or take. All three of these numbers compare an individual tire to a standard one the Department Of Transportation uses. The DOT Standard tire is not something you can buy, nor would you want to; it is simply a yardstick used to compare other tires.
Temperature is the tire's resistance to building up heat as it rolls and flexes. The less heat it generates, the longer it will last.
Traction is how well the tire grips in straight-line braking. This is mostly a measure of the tread compound's grip and should not be confused with cornering traction (how fast you can go around a curve), which is a much more complicated subject; one that depends in part on the car's suspension tuning.
Tread Ware is a percentage of how quickly your tire will wear out, versus the standard tire. If the standard tire will go 15,000 miles before the tread is gone, then a tire with a 150 rating will go 50% (7,500 miles) farther before it needs replacing. Assuming, of course, you don't go playing "road racer" or doing smoking burn-outs at every stop light.
For more information and a further explanation of what all those terms -- and many more -- mean, click HERE.