How to read tire size codes
Decode the letters and numbers on your tire sidewall so you know exactly what fits your car and what each part of the rating means.
A code like P225/65R17 100T tells you everything you need to match a replacement tire: vehicle class, width in millimeters, sidewall height ratio, construction, wheel diameter, load capacity, and top speed rating. Read it left to right and the tire sidewall stops looking like alphabet soup.
What each section of the code means
Take P225/65R17 100T as the working example. Every passenger car tire follows the same pattern.
| Code part | Example | Meaning |
|---|---|---|
| Service letter | P | Passenger car (LT = light truck, ST = trailer, T = temporary spare, C = commercial) |
| Section width | 225 | Sidewall-to-sidewall width in millimeters |
| Aspect ratio | 65 | Sidewall height as a percent of width, so 146 mm here |
| Construction | R | Radial (D = bias, B = belted bias, F-prefix = run-flat) |
| Rim diameter | 17 | Wheel diameter in inches |
| Load index | 100 | Max load per tire (100 = 1,764 lb) |
| Speed rating | T | Max sustained speed (T = 118 mph) |
Tires with no service letter at the front are typically European metric, also called Euro-metric. They use the same width/ratio/diameter rules but are loaded slightly differently than P-metric tires of the same size.
Service letter quirks worth knowing
LT tires take higher inflation pressures (often 65 to 80 psi) and carry load ranges like C, D, or E. If you swap a P-metric tire for an LT tire of the same numerical size on a half-ton truck, you usually lose a little load capacity unless you keep the pressure up.
ST tires are trailer-only. They have stiffer sidewalls for vertical load and are not rated for steering. Putting an ST on the front of a tow vehicle is asking for a sidewall failure.
The T-rated temporary spare is the donut. Most are speed-limited to 50 mph and rated for about 50 miles of use.
How to read the load index
The load index is not pounds. It maps to a number on a standardized table. A few common reference points:
- 91 = 1,356 lb
- 95 = 1,521 lb
- 100 = 1,764 lb
- 105 = 2,039 lb
- 110 = 2,337 lb
- 117 = 2,833 lb
- 121 = 3,197 lb
Multiply by four to get the vehicle’s tire-rated weight capacity. The number you actually care about is on the door jamb placard, which lists OE tire size and the load index your car was certified with. Going below that number is a safety problem and in most states a violation.
How to read the speed rating
The letter at the end caps the sustained top speed. Common ones for passenger cars and light trucks:
| Rating | Top speed |
|---|---|
| S | 112 mph |
| T | 118 mph |
| H | 130 mph |
| V | 149 mph |
| W | 168 mph |
| Y | 186 mph |
| (Y) | 186+ mph |
H is the most common factory fit on family sedans and crossovers. SUVs and trucks usually run T or H. Performance cars get V, W, or Y.
You can go up in speed rating (T to H, H to V) without trouble. Going down (H to T) can affect handling, the speedometer is unaffected, and some insurers consider it a downgrade.
Where to find the right size for your car
The door jamb sticker on the driver’s side is the source of truth. It lists OE tire size, recommended inflation pressure (cold), and the load index the vehicle was certified with. The owner’s manual lists the same data, often with winter or optional sizes.
The numbers on your current tires only help if those tires are the correct size. People put oddball tires on used cars all the time. Cross-check against the placard before ordering.
Plus-sizing without breaking things
If you want bigger wheels, the rule is to keep the outside diameter within about 3 percent of stock. Drop the aspect ratio by 10 for every inch you add to the rim:
- Stock: 225/65R17
- Plus one: 235/60R18
- Plus two: 245/55R19
Width and load index both have to stay at or above OE. The closer you stay to the original rolling diameter, the less your speedometer and ABS calibration drift.
DOT date and other sidewall marks
The four digits at the end of the DOT code are the build date. 2724 means week 27 of 2024. Tires older than six years from manufacture should be inspected annually regardless of tread depth, and most manufacturers recommend retirement at ten years even if the tread looks fine. Rubber ages.
“M+S” or the three-peak mountain snowflake indicates winter capability. M+S alone is just mud and snow tread pattern. The snowflake symbol is the actual cold-weather certification.
Treadwear, traction, and temperature numbers (UTQG) sit on the sidewall too. Treadwear is a comparative number, not a mileage estimate, and it is only meaningful within one manufacturer’s lineup.