The short answer

The tire sidewall identifies its size, construction, and other ratings. The vehicle’s label and manual tell you the approved tires and recommended cold pressure. These are related sources of information, but they do different jobs.

Read a size one part at a time

Consider the example 205/55 R16. The 205 is the nominal tire width in millimeters. The 55 is the aspect ratio: the sidewall height as a percentage of that width. The R identifies radial construction. The 16 describes the wheel diameter in inches at the tire’s mounting seat.

These numbers combine different units. The 55 is not 55 millimeters, and R does not mean radius. Other markings identify load and speed ratings. A load index needs its corresponding table; it is not a weight written directly in kilograms. A speed rating is not permission to drive at that speed on a public road.

Sources: Bridgestone Americas ↗

Example size 205/55 R16: nominal width 205 millimeters, height-to-width ratio 55 percent, radial construction, and 16-inch wheel diameter.
Original diagram · simplified, not to scale. About our diagrams

Use the vehicle’s pressure recommendation

Look for the Tire and Loading Information Label, commonly on the driver’s door opening in U.S.-market vehicles. The owner’s manual also identifies the required size and inflation guidance. Check pressure when the tires are cold, following the stated procedure. Front and rear recommendations may differ.

A maximum-pressure marking on a tire is not generally the recommended operating pressure for your car. Do not set all four tires to that number simply because it is molded into the sidewall. A tire pressure monitoring system, or TPMS, is useful, but routine gauge checks and damage inspections still matter.

Sources: National Highway Traffic Safety Administration ↗

What this means when buying

Compare the installed tires with the manufacturer’s requirements for that configuration. Ask an inspector about mismatched sizes, unsuitable load ratings, uneven wear, damage, and replacement needs. Some cars intentionally use different front and rear sizes. A difference needs checking rather than an automatic assumption of error.

Look at tire age as well as tread. On relevant U.S. tire markings, the final four digits of the tire identification number give the week and year of manufacture. A fictional ending of 1925 means week 19 of 2025. Follow vehicle and tire manufacturer guidance on age and replacement; this date code is not a universal expiry schedule.

Sources: National Highway Traffic Safety Administration ↗ · Bridgestone Americas ↗

Common misunderstandings

Two tires with the same size code are not necessarily interchangeable in every application. Confirm the required type, load rating, and other manufacturer specifications before replacing them.

A pressure warning going out does not prove the tire has no damage or slow leak. Have unexplained pressure loss assessed. Inflation, tread condition, and age all contribute different evidence about whether a tire remains suitable.

Sources: National Highway Traffic Safety Administration ↗ · Bridgestone Americas ↗

WORDS YOU NOW KNOW
Tire aspect ratio Tire load index Tire pressure monitoring system (TPMS) Traction

Sources & review

Reviewed . Original educational explanation, checked against the sources below. This is general guidance; consult the documents for the exact vehicle, year, configuration, and market.

  1. Determining Tire SizeBridgestone Americas · Width; Aspect Ratio; Construction; Rim Diameter; Load Index; Speed Rating · Accessed 2026-10-22.
  2. Tire Safety Ratings and Awareness · TireWiseNational Highway Traffic Safety Administration · Tire Maintenance; Tire Pressure; Aging · Accessed 2026-10-22.
Editorial policy & corrections →
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What comes next?

The four systems that keep you in controlFront-wheel, rear-wheel, or all-wheel drive?What maintenance does a car need—and why?