The right pressure
The number molded into the tire's sidewall is the maximum the tire can hold, not the pressure your car wants. The correct figure is on the placard inside the driver's door jamb, usually between 30 and 36 pounds per square inch. Measure when the tires are cold, before driving, because a warm tire reads several pounds high. Tires lose about one pound per month on their own and another pound for every ten-degree drop in temperature, which is why the warning light tends to appear on the first cold morning of autumn.
Tread and when to replace
A new tire has about 10/32 of an inch of tread. The penny test marks 2/32, which is the legal minimum in most states and the point at which the tire is already finished. Wet grip falls off well before that, so use a quarter as the earlier warning: if the tread does not reach the top of Washington's head, you are at 4/32 and should start shopping. Wear bars, the raised bridges across the grooves, sit flush with the tread when the tire is done.
Reading the sidewall
A size such as 215/60R16 95H is less cryptic than it looks.
- The first number, 215, is the width of the tire in millimeters.
- The second number, 60, is the height of the sidewall as a percentage of the width.
- The letter R means radial construction, and 16 is the wheel diameter in inches.
- The final pair, 95H, gives the load index and the speed rating, and a replacement should match or exceed both.
Look for the letters DOT followed by a string ending in four digits. Those digits give the week and year of manufacture, so 2321 means the twenty-third week of 2021. Rubber hardens with age, and most manufacturers advise replacing any tire at six to ten years old, whatever the tread looks like.
What the wear tells you
Wear on both outer edges points to chronic underinflation, and wear down the center points to overinflation. Wear on one edge only means the alignment is off, and a scalloped, cupped pattern suggests worn shocks or struts. Replace tires in pairs at the least, and put the new pair on the rear axle, where grip keeps the car from spinning in the wet. Many all-wheel-drive cars require four closely matched tires, so check the manual before replacing only two.
Tip Buy a digital gauge for about ten dollars and keep it in the glove box. Gas station gauges are dropped, run over and frequently wrong.
