Every tire has a string of letters and numbers stamped on its sidewall that looks intimidating but is actually a complete, standardized spec sheet once you know how to read it. Understanding it means you can confirm a replacement tire is genuinely correct for your car, not just close enough.
Breaking down an example: P215/65R16 91H
- P — tire type. "P" means passenger vehicle; you might also see "LT" for light truck.
- 215 — width in millimeters, measured sidewall to sidewall.
- 65 — aspect ratio, the sidewall height as a percentage of the width. Lower numbers mean a shorter, stiffer sidewall.
- R — construction type. "R" means radial, which is what nearly every modern tire uses.
- 16 — wheel diameter in inches that the tire is designed to fit.
- 91 — load index, a coded number representing the maximum weight the tire can safely carry.
- H — speed rating, a letter code for the maximum sustained speed the tire is rated for.
Why the load index and speed rating actually matter
It's tempting to treat these as fine print, but going below your vehicle manufacturer's specified load index or speed rating is a genuine safety issue, not just a technicality — it means the tire wasn't engineered to handle what your specific vehicle might ask of it. Going above spec is generally fine; going below isn't recommended.
Where to find what your car actually needs
The manufacturer's recommended tire size and ratings are printed on a sticker inside the driver's door jamb, not just in the owner's manual — that sticker reflects the exact spec for your trim, which can differ from what's printed elsewhere for the model line in general.
Bringing this information with you (or just knowing how to read your own sidewall) makes it much easier to have a precise conversation with a tire shop about exactly what you need, rather than relying entirely on their lookup.