Tire Size Decoder

Take 215/60R16 95H. 215 is the width across the tire in millimetres. 60 is the aspect ratio — the sidewall is 60% of that width, so 129 mm tall. R means radial construction. 16 is the wheel diameter in inches, measured at the bead seat. 95 is the load index, which is 1,521 lbs per tire. H is the speed rating, certified to 130 mph. The first four have to be right for the tire to physically fit; the last two have to meet or beat what your car was built for.

Open the driver's door and look at the placard on the door jamb. That is the size, load index and speed rating the car was engineered around, along with the correct inflation pressures. The owner's manual repeats it. The sidewall of the tires currently on the car tells you what is fitted, which is not always the same thing — a previous owner may have changed it. When the placard and the sidewall disagree, the placard is the one to trust.

Physically, often yes. Whether you should is a different question. Most fitment guides work to within 3% of the original overall diameter, because your speedometer, odometer, ABS and traction control all read wheel speed and assume the original rolling circumference. Past that they are all reading wrong by the same margin. A replacement also has to meet or beat the original load index, and it has to clear the suspension and bodywork at full steering lock and full suspension travel — which is something no calculator can see. Check the owner's manual before you buy.

Some cars leave the factory staggered, with a wider tire on the rear — that is fine, because it is the design. Mixing sizes that were not designed to be mixed is not. Never mix sizes across one axle. On an all-wheel-drive car, even small differences in rolling circumference between axles make the centre differential or coupling work continuously, and that gets expensive. Replace in pairs across an axle at minimum, and all four on an all-wheel-drive car.

Plus-sizing means going up in wheel diameter and down in sidewall to keep the overall diameter roughly the same — 205/55R16 to 225/45R17, for example. Less sidewall flex sharpens steering response and usually looks better. You also get a harsher ride, more road noise, more exposure to pothole and rim damage, and a more expensive tire to replace. It needs the matching wheel: the tire cannot go on the wheel you have.

No, and this is the single most common misreading of a tire size. The aspect ratio is a percentage of that tire's own width, not a fixed measurement. A 205/60R16 has a 123 mm sidewall; a 275/60R16 has a 165 mm one. Same 60, 42 mm difference.

A letter in front — P, LT or ST — is the service type: passenger, light truck, or special trailer. XL or Reinforced after the size means the tire is built to carry more than a standard tire of the same size, and it will carry a higher load index to match. Both matter for load, neither changes the tire's dimensions.