Every tire on the road carries its complete resume molded into the rubber — size, speed capability, load limits, age, even which direction it’s supposed to spin. Once you know how to read a tire sidewall, you can spot an unsafe tire in a parking lot, catch a shop installing the wrong spec, and decode exactly what you’re buying before the money leaves your account. Here’s the full guide, from the big size numbers down to the fine print most drivers never notice.
The Size Code: 245/40R18 Decoded
- 245 — the tire’s section width in millimeters, sidewall to sidewall.
- 40 — the aspect ratio: sidewall height as a percentage of the width (here, 40% of 245mm ≈ 98mm). Lower numbers mean a shorter, stiffer sidewall — sharper steering response, harsher ride, and more vulnerability to pothole damage.
- R — radial construction, standard on virtually everything modern. (You’ll occasionally see ZR on performance tires — a legacy marking that means “speed rating above V,” with the actual rating in the service description.)
- 18 — wheel diameter in inches.
A P prefix (P245/40R18) means P-metric passenger spec; LT means light truck construction with higher load capability at higher pressures — and the two are not interchangeable on trucks that specify LT.
Load Index and Speed Rating: The Numbers That Keep You Alive
The service description follows the size — for example, 94Y. The number is the load index (per-tire weight capacity from a standardized chart), the letter is the maximum sustained speed rating:
| Load Index | Per-Tire Capacity | Speed Rating | Max Sustained | |
|---|---|---|---|---|
| 91 | 1,356 lbs | S | 112 mph | |
| 94 | 1,477 lbs | T | 118 mph | |
| 98 | 1,653 lbs | H | 130 mph | |
| 100 | 1,764 lbs | V | 149 mph | |
| 104 | 1,984 lbs | W | 168 mph | |
| 108 | 2,205 lbs | Y | 186 mph |
An XL (Extra Load) marking means the tire achieves a higher load index at higher inflation pressure — common on heavy EVs and performance sedans. The rule that matters: always replace tires with an equal or higher load index and speed rating than your vehicle’s placard specifies. An underrated tire can overheat and fail under sustained high-speed or high-load conditions — it’s a safety spec, not a suggestion.
The DOT Code: How Old Is This Tire, Really?
Near the bead you’ll find “DOT” followed by a string of characters ending in four digits — the week and year of manufacture. A tire stamped 2324 was built in the 23rd week of 2024. This is the most valuable number on the sidewall when buying anything used, checking a “new” tire that’s been on a warehouse shelf, or evaluating the spare you’ve never looked at: rubber degrades with age regardless of tread, and tires past roughly six years are generally considered at the end of their safe service life — ten years is a hard ceiling even for a never-used spare. If a shop installs tires with date codes more than a year old and charged you new-tire prices, that’s a conversation worth having.
UTQG: Treadwear, Traction, Temperature
Printed separately on the sidewall, the UTQG rating has three parts. Treadwear is a relative longevity number — a 400 tire should outlast a 200 tire, but only within the same manufacturer’s lineup, since everyone runs their own baseline (this is also your quickest read on a tire’s character: a 200-treadwear tire is a grippy, fast-wearing performance compound; a 700 is a long-life tourer). Traction (AA, A, B, C) grades wet braking, and temperature (A, B, C) grades heat resistance at speed. For anything sporty, A/A is the floor you want.
The Fine Print Worth Knowing
- M+S vs. the mountain-snowflake (3PMSF): M+S is a mud-and-snow tread geometry claim with no performance test behind it. The three-peak mountain snowflake symbol means the tire passed an actual standardized snow traction test. If winter matters where you live, the mountain is the mark that counts.
- Rotation and mounting arrows: directional tires carry an arrow — mounted backward, they’re measurably worse in rain. Asymmetric tires say “OUTSIDE” on the face that must face out. Worth checking after any tire shop visit; mistakes happen weekly.
- OE fitment marks: a star (BMW), MO (Mercedes), N-number (Porsche), AO (Audi), or TO (Tesla) means the tire was built to that automaker’s spec. Matching them isn’t mandatory, but mixing OE-marked and standard versions of the “same” tire on one axle can produce odd handling.
- Max pressure ≠ recommended pressure: the sidewall’s maximum pressure is the tire’s structural limit, not a target. Your correct pressures live on the driver’s door jamb placard.
- Run-flat markings: RFT, ROF, SSR, or EMT depending on brand — relevant because run-flats generally can’t be repaired after running deflated, and mixing run-flats with conventional tires is a bad idea.
Putting It to Work
Five minutes with this knowledge covers you at every tire touchpoint: verify the date codes and specs on any tire quote before approving it, check the load/speed ratings match the placard when buying online, read the wear and age on a used car’s tires as evidence of how it was maintained, and don’t forget the spare — it has a date code too, and it’s probably older than you think (here’s how to change a flat when you finally need it). New set going on? Break them in properly before pushing hard. For deeper spec research by model, Tire Rack remains the reference.
Recommended Gear
Once you know how to read a tire sidewall, it’s worth checking those numbers regularly. A few tools make that easy:
- Tire Tread Depth Gauge — quickly checks remaining tread against the wear bars
- Digital Tire Pressure Gauge — confirms you’re running the placard PSI, not the sidewall maximum
- Tire Sidewall Protectant — helps keep sidewall rubber from cracking prematurely in sun-heavy climates
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