What do the numbers on a tire mean?

Read a size like P225/65R17 in pieces. P means passenger vehicle, with LT indicating light truck. 225 is the section width in millimeters across the tread. 65 is the aspect ratio, meaning the sidewall height is 65 percent of the width, so a lower number means a shorter, stiffer sidewall. R means radial construction, standard on modern tires. 17 is the wheel diameter in inches, an inherited unit that makes the code awkwardly mixed. Following that you will typically see a load index and a speed rating letter, which specify the maximum weight and sustained speed the tire is engineered to handle, and going below your vehicle's specification is genuinely unsafe.

Two other sidewall markings matter. The DOT code ends in four digits giving the week and year of manufacture, so 3223 means the 32nd week of 2023, and this matters because rubber degrades with age regardless of tread depth, with most manufacturers recommending replacement at six to ten years even on a tire that looks new. The UTQG grades for treadwear, traction, and temperature let you compare within a brand, though the treadwear number is self-reported and not reliably comparable across manufacturers. The correct sizes for your vehicle are on a placard inside the driver's door jamb, which is also where you find the recommended inflation pressure, and that placard, not the number molded on the tire sidewall, is the pressure you should be running.