The short answer
A tire size like P225/65R17 102H breaks into five parts: P = passenger vehicle, 225 = section width in millimeters, 65 = aspect ratio (sidewall height is 65% of the width), R17 = radial construction on a 17-inch rim, and 102H = load index and speed rating. The correct size for your vehicle is on a sticker inside the driver's door jamb — not on the tire itself, which may have been changed by a previous owner.
What do the numbers in a tire size like 225/65R17 mean?
Starting from the left: 225 is the section width in millimeters, 65 is the aspect ratio so the sidewall height is 65% of 225 mm, or about 146 mm, R means radial construction, and 17 is the wheel diameter in inches the tire fits. So 225/65R17 is a 225 mm wide radial tire built for a 17-inch rim.
| Part | Example | What it means |
|---|---|---|
| Section width | 225 | Tread width across the tire, in millimeters |
| Aspect ratio | 65 | Sidewall height = 65% of 225 mm (≈146 mm) |
| Construction | R | Radial — the standard for modern passenger tires |
| Rim diameter | 17 | Wheel diameter in inches the tire is designed to fit |
The full string P225/65R17 is molded into every tire's sidewall. Tire Rack illustrates each marking on the sidewall, and you can compare it against the OEM spec on your door-jamb sticker.
What does the letter before the numbers (P or LT) tell you?
P (passenger) is the most common prefix, meaning the tire meets U.S. standards for passenger-car loads. LT (light truck) tires are built for heavier loads and higher inflation pressures, used on pickups and heavy SUVs rated to tow or haul. If your door-jamb sticker specifies LT, you must use LT — substituting P-metric tires risks an underload failure.
- P (P-metric): passenger-car rated; the default on most sedans, crossovers, and minivans.
- LT (light truck): heavier load capacity and higher max pressure for towing and hauling. Running P tires where LT is required is a common, dangerous mistake on trucks.
- No prefix (Euro-metric): similar sizing math but a different load standard than P-metric — check the load index, not the prefix, to confirm capacity.
What are tire load index and speed rating?
The load index (e.g., 102) is a numerical code for the maximum weight one tire can support: load index 102 equals 1,874 lbs per tire, so a set supports up to 7,496 lbs. The speed-rating letter (e.g., H) marks the maximum sustained speed — H = 130 mph, V = 149 mph, W = 168 mph. Never fit a lower speed rating than the manufacturer specifies.
| Speed rating | Max sustained speed |
|---|---|
| H | 130 mph |
| V | 149 mph |
| W | 168 mph |
The speed rating is a safety ceiling, not a driving goal — it reflects the heat the tire is built to withstand. Matching the manufacturer's load index also keeps your tire and maintenance budget aligned with the right replacement, since the wrong rating can void coverage.
What is the DOT code on a tire sidewall, and how do you read a tire's age?
The DOT code is a string beginning with "DOT" on the sidewall, and its last four digits encode the manufacture date: the first two are the week (01–52) and the last two are the year. "2423" means the 24th week of 2023. NHTSA notes rubber degrades from UV and heat even on low-mileage tires, so check the DOT date before buying used.
- Find the string starting with "DOT" on the sidewall.
- Read the last four digits as week + year — "2423" = week 24 of 2023.
- Many automakers and NHTSA guidance recommend replacing tires older than 6–10 years regardless of tread depth.
- Always check the date code when buying used tires — and pair it with the tread checks in our guide on when to replace your tires.
Do all tires on your car have to be the same size?
Yes, with one exception: spare tires are typically smaller "donut" spares rated for temporary use only, usually a 50 mph max and a 50-mile limit. For all four main tires, the manufacturer's specified size is required so the speedometer, ABS, stability control, and traction control read wheel speed accurately. Changing to a meaningfully different diameter throws off all of these systems.
- Four main tires: match the door-jamb spec — mismatched diameters confuse ABS and stability control.
- Spare: a temporary "donut" is the only intentional exception; observe its 50 mph / 50-mile limit.
- Symptoms of a mismatch: uneven wear or a pull can mimic alignment issues — see why a car pulls to one side and keep tires even with regular rotation.
Sticking to the door-jamb specification is the safe and legal choice, and it keeps your alignment working as designed.
Frequently asked questions
Where do I find the correct tire size for my car?
The authoritative source is the sticker on the inside of the driver's door jamb. It lists the OEM tire size, the recommended cold inflation pressure, and the load rating. You can also find it in the owner's manual. Do not rely solely on what's currently on the car — tires may have been changed by a previous owner.
What does "plus sizing" a tire mean?
Plus sizing (or "upsizing") means fitting a larger-diameter wheel with a lower-profile tire to keep the overall diameter the same. For example, moving from 225/65R17 to 245/45R19. The wheel gets bigger, the sidewall gets shorter, and the section width gets wider. It affects ride comfort (stiffer), handling (sharper), and requires verifying clearance — not all sizes fit every car.
Can I use a wider tire than what's recommended?
Within limits, yes — many drivers go one size wider for aesthetics or traction. But too wide a tire won't fit the wheel properly and can rub the wheel well, suspension, or body panels under full steering lock or suspension compression. Check fitment guides at Tire Rack or your tire shop before ordering.
What does the UTQG treadwear number mean?
UTQG (Uniform Tire Quality Grading) is an NHTSA standardized rating system. The treadwear number (e.g., 500) is a relative index — a tire rated 500 should last twice as long as a baseline tire rated 100 under the same test conditions. It's useful for comparing tires from the same brand; cross-brand comparisons are less reliable because each manufacturer runs its own baseline.
Sources
CarsLens is editorial guidance, not individualized advice. This page draws on Tire Rack and NHTSA.