How to Read Your Sidewall

Every tire has its size and specs molded into the sidewall. Once you know what each number means, you can compare any brand and check that a size really fits your car. Here is a typical performance size, broken down:

The size

• 245 – Section width in millimeters, sidewall to sidewall.
• 40 – Aspect ratio. The sidewall is 40% as tall as the tire is wide (about 98mm here).
• ZR – Radial construction rated above 149 mph. Most sizes just show R.
• 18 – Wheel diameter in inches. It has to match your wheel.

Changing wheel size? Keep the overall diameter within about 3% of factory so your speedometer, ABS and AWD stay happy. The Fitment Finder tire tab does that math for your exact car, front and rear.

Service description

• Load index (97) – How much weight each tire can carry. See the load index chart.
• Speed rating (Y) – The max sustained speed the tire is rated for. See speed ratings.
• XL / Extra Load – A reinforced tire that carries more weight at higher pressure. Common on heavier sedans, EVs and staggered rear sizes.

Other markings

• OE markings – Tires built to a car maker's spec carry a code: * (star) for BMW, MO for Mercedes, AO / RO1 for Audi, N0-N4 for Porsche.
• RFT / SSR / ZP – Run-flat construction, standard on many BMW and Mini models.
• DOT code – The last four digits are the week and year it was made. 2324 = 23rd week of 2024.
• Max pressure – The most air the tire can safely hold. Not what you run day to day; use the placard in your door jamb.
• Directional / Outside – Arrows or "outside" markings show how the tire must be mounted.
• 3-Peak Mountain Snowflake – Meets a severe-snow traction standard.
• UTQG (e.g. 300 AA A) – Treadwear, traction and temperature grades. Summer performance tires often run 200-300 treadwear; track tires go lower.