Look at the outside sidewall of any tire and you'll see a string of numbers and letters that looks like alphabet soup. It's actually a precise specification for your tire's size, construction, and speed/load rating. Knowing how to read it saves you time and money the next time you need new rubber.
The full code, decoded
Take a typical tire marking: P245/55R18 102H.
- P — Tire type. P = passenger, LT = light truck, ST = trailer. No letter usually means metric (Euro) sizing.
- 245 — Section width in millimeters. Measured sidewall to sidewall when mounted.
- 55 — Aspect ratio. Sidewall height as a % of width. Lower number = shorter sidewall = sportier feel.
- R — Construction type. R = radial (modern). Almost everything on the road today.
- 18 — Wheel diameter in inches. The tire fits an 18-inch rim.
- 102 — Load index. 102 ≈ 1,874 lbs per tire. Higher number = more weight.
- H — Speed rating. H = up to 130 mph. Higher letters (V, W, Y) handle faster speeds.
Why this matters when you replace
Putting the wrong size tire on your car will throw off your speedometer, ABS, traction control, and (on some vehicles) tire-pressure monitoring. It can also cause the tire to rub against fender liners or suspension parts at full lock or full compression.
When you upgrade to bigger tires (e.g. on a lifted truck), the goal is to keep the overall rolling diameter close to stock — or compensate via re-gearing or a tuner. We work that math out for free when you bring your truck in.
How to find your size
- Look at any current tire's sidewall — easiest if you can find them all in matching sizes.
- Check the placard on the driver's door jamb — every car has one.
- Open the owner's manual to the tire section.
- Or just call us and tell us your year/make/model — we'll look it up.
Bring the vehicle in or call (916) 627-1998 — we keep hundreds of common sizes in stock and can special-order anything we don't have.