Slowly, but with the certainty of tomorrow's sunrise, we are going metric.
The metric system also encompasses tires. Used to be tires were labeled under what was knownas the numeric system. Then slowly the tire industry switched over to what is called the alphanumeric system. Now the switch is toward what the tire industry calls the P-Metric system.
Many new tires are being made with the new metric sizes. Goodyear's new all-season Tiempo, one of the fastest selling tires in history, uses metric sizes. Many new cars are equipped with metric-size tires. Here's how to understand the system.
Let's take a common metric tire size - P205/75R14. The first letter in the metric system will always indicate the family of tires. In this case, "P" indicate that the tire is for passenger cars. In fact, right now there are no truck tires made that are labeled with the metric system - although that will happen. When it does, the first letter (or letters)of the label will be something other than "P," to indicate truck tires. Right now the tire industry has not agreed on what it will be.
The next group of numbers, "205" in this case, is what the tire people call the "section width." That's the distance across the widest part of the tire, from one sidewall to the other. So 205 means that the distance from one sidewall to the other is 205 millimeters.
The two numbers after the slash refer to the "aspect ratio," the distance from the bottom of the tire beadto the top of the tire tread, divided by the section width, and expressed in percent. The tire bead is that lip around the inside that fits snugly against the rim. Aspect ratio is always expressed as a percentage. So "75" means that the distance from the bottom of the tire bead to the top of the tire tread is 75 percent of the distance across the widest part of the tire.
Section width on thistire is 205 millimeters, so if we take out our pocket calculator, we find that 75 percent of 205 is 153.75. That means the distance from the bottom of the tire bead to the top of the tread is 153.75 millimeters.
The next position in the labeling sequence designates the type of tire. "R" stands for radial, "B" for bias-belted, and "D" for bias-ply. So in our example, the R means the tire is a radial.
The last number indicates the diameter of the rim. In this case, "14." That means the rim is a 14-inch rim, in other words, it's 14 inches in diameter.
Let's take another common metric tire size, the one found on all new Oldsmobile Cutlas V8s - P195/75R14. That means the tire is a passenger tire, it's 195 millimeters wide at its widest point, its height from the bottom of the bead to the top of the tread is 75 percent of its section width, which translates to 146.24 millimeters. It's a radial tire, and it's on a 14-inch rim.
One other thing. With the tires that we are used to now, you can get three load ranges - B, C, or D. With the new metric tires only two load ranges will be available - Standard Load, and Extra Load. The standard load tire can be inflated up to 35 psi, and the extra load tire can be inflated to a maximum of 41 psi. If you carry heavy loads most of the time, you would want the extra loads tires. If you carry light loads most of the time, stick with the standard load tires.
With P-Metric radials, some experts recommend that whether you have the standard load tire or the extra load tire, that you keep them inflated to their maximum pressures - even if you aren't carrying a load. Some insiders I've talked to at one of the major tire companies say they have done tests with skilled drivers, inflating the tires to their normal recommended pressures, and then their highest recommended pressures, and the drivers were unable to consistently detect the difference in ride comfort (with bias tires, high inflation pressures definitely give a harsher ride).