Decoding a tire sidewall is one of those skills that suddenly makes buying tires, checking specs, and understanding your vehicle dramatically less confusing. Once you know what the markings mean, you can read a tire like a label — and verify what you are getting before it is mounted.
The size string — the most important part
Example: 225/65R17 or P225/65R17. Every passenger and light truck tire has a size string.
P (or no prefix): "P" means P-metric, designed for passenger vehicles. "LT" means Light Truck — heavier construction, higher load capacity. No prefix is common on European-metric tires.
225: section width in millimeters — the width of the tire from sidewall to sidewall at its widest point.
65: aspect ratio — the height of the sidewall as a percentage of the section width. A 225/65 tire has a sidewall that is 65% of 225mm = 146mm tall.
R: radial construction — nearly all tires on modern vehicles are radial. Bias-ply tires still exist for certain applications.
17: wheel diameter in inches — the size of the rim the tire fits.
Load index
The load index is a number (usually 2-3 digits, like 91 or 115) that appears after the size string.
Load index 91 = 1356 lbs max per tire. Load index 115 = 2679 lbs per tire. Higher number = heavier load capacity.
Common passenger tire load indexes: 89 (1279 lb), 91 (1356 lb), 94 (1477 lb), 97 (1609 lb), 100 (1764 lb), 103 (1929 lb).
For trucks and SUVs: the vehicle's door jamb sticker lists the minimum load index required. Never install a tire with a lower load index than specified — it is a structural capacity issue, not just a legal one.
Speed rating
A single letter follows the load index: T, H, V, W, Y are the most common on modern vehicles.
T = 118 mph. H = 130 mph. V = 149 mph. W = 168 mph. Y = 186 mph.
The speed rating defines the maximum sustained speed the tire can safely handle under its maximum load. It is an engineering specification — not a recommendation to drive at those speeds.
You can uprate speed (e.g., H to V) without any issue. You cannot downrate — a W-rated vehicle should not get T-rated tires.
Performance vehicles often specify W or Y minimum. Most daily drivers specify H or V.
DOT code and manufacture date
Every tire has a DOT code starting with "DOT" on the sidewall. The last four digits are the most important: week and year of manufacture. "2322" means week 23 of 2022.
Tire age matters: most manufacturers recommend replacing tires 6–10 years from manufacture regardless of tread depth. Rubber compounds degrade even when the tire looks fine.
When buying tires, especially "new" tires at a discount shop, check the DOT date. A tire manufactured 4+ years ago may have significant shelf life already consumed.
UTQG ratings
UTQG (Uniform Tire Quality Grading) is a government-mandated rating system printed on the sidewall: three numbers like 500 A A or 300 A B.
First number — Treadwear: relative tread life compared to a government reference tire. 400 lasts roughly twice as long as 200. Not a mileage guarantee — a relative index.
Second letter — Traction: how well the tire stops on wet pavement. AA is the highest. A is good. B is adequate. C is minimum.
Third letter — Temperature: heat resistance. A (highest), B, C (minimum legal). Most quality tires rate A.
Frequently asked
Can I put a different size tire on my car?
Minor plus-sizing (same diameter, wider width) is common for aesthetics or performance. Significantly different diameters affect speedometer accuracy, ABS calibration, and traction control — usually within 3% of the original diameter is the guideline. Always verify fitment with the shop.
What does M+S mean on a tire?
M+S (Mud + Snow) means the tread meets a minimum threshold for mud and snow traction. It is a self-designation by the tire manufacturer — not an independent test. It does NOT mean the tire is a true winter tire. Look for the Three-Peak Mountain Snowflake (3PMSF) symbol for independently verified winter performance.
Keep reading
Last updated 2026-06-27. General guidance only — confirm specifics with a local shop for your exact vehicle.