How to Read Tire Size:
What Do the Numbers Mean?

Every tire has a size code molded into its sidewall — a string of numbers and letters like 205/55R16. Once you know what each part means, you can read any passenger tire size in the world.

205/55R16

A typical passenger car tire size

205
Section Width
The tire's width in millimeters, measured from sidewall to sidewall at the tire's widest point when mounted and inflated. Common values: 155–335 mm.
55
Aspect Ratio
Sidewall height as a percentage of the section width. A 55-series tire has sidewalls that are 55% of 205 mm = 112.75 mm tall. Lower number = shorter, sportier sidewall.
R
Construction
R stands for Radial — the cord plies run radially across the tire. Nearly all modern passenger tires are radial. You may also see D (diagonal/bias-ply) on older vehicles.
16
Rim Diameter
The diameter of the wheel the tire is designed to fit, in inches. Common sizes: 14–22 inches. This must match your wheel exactly.
Side view Front view 205 mm Section Width 55% Sidewall height (112.75 mm) R16 (16 inches) Rim diameter = 406.4 mm 205/55R16
Overall Diameter Formula:
Diameter = (Width × Aspect Ratio% × 2) + (Rim inches × 25.4)
For 205/55R16: (205 × 0.55 × 2) + (16 × 25.4) = 225.5 + 406.4 = 631.9 mm (24.88 in)

What does 205 mean on a tire?

The first number in any tire size code is the section width — the distance across the widest point of the tire from one sidewall to the other, measured in millimeters. For a 205 tire, that's 205 mm (about 8.1 inches) wide.

This is measured on an inflated tire mounted on its specified rim width. On a wider rim, the tire spreads out slightly; on a narrower rim, it narrows. Common widths for passenger cars range from 155 mm to 255 mm.

What does 55 mean on a tire?

The second number is the aspect ratio — the sidewall height expressed as a percentage of the section width. A 55-series tire has sidewalls that are 55% of 205 mm = 112.75 mm tall on each side.

A lower aspect ratio (e.g., 35 or 40) means a shorter, stiffer sidewall. These are common on performance and sport vehicles and give better cornering response. A higher ratio (e.g., 65 or 70) means a taller sidewall, which absorbs bumps better — common on trucks, SUVs, and economy cars.

What does R mean on a tire?

R stands for Radial construction. In a radial tire, the cord plies run perpendicular (radially) to the direction of travel, which makes the tire more flexible and fuel-efficient. Virtually all modern passenger, light truck, and SUV tires are radial.

Older vehicles may use D (diagonal/bias-ply) tires, where cords run at an angle. You should never mix radial and bias-ply tires on the same axle.

What does 16 mean on a tire?

The last number is the rim diameter in inches. This must exactly match the diameter of your wheel. A 16-inch rim is 16 inches across the bead seat — approximately 406.4 mm. Common sizes range from 14 to 22 inches.

Never try to mount a tire with a different rim diameter than your wheel. A 15-inch tire will not seat on a 16-inch rim.

Can I use a different tire size?

You can typically use a size that keeps the overall diameter within ±3% of the original. Changing the overall diameter affects your speedometer, odometer, ground clearance, and potentially your ABS calibration.

A common reason to change size is to fit larger wheels (plus-sizing) — going from a 16" rim to a 17" or 18" rim while reducing the aspect ratio to keep the same overall diameter. Use our Tire Size Comparison Calculator to see the exact difference before making changes.

Frequently Asked Questions

The three numbers in a tire size code tell you the tire's width in mm, the sidewall height as a percentage of that width, and the rim diameter in inches. For example, 225/60R17 means 225 mm wide, sidewalls 60% of 225 mm tall, fits a 17-inch rim.
205 is the section width in millimeters — the measurement across the widest point of the inflated tire from sidewall to sidewall. In inches, that's about 8.1 inches.
55 is the aspect ratio — the sidewall height as a percentage of the section width. For a 205/55 tire, the sidewall height is 55% × 205 mm = 112.75 mm (about 4.4 inches) on each side.
R = Radial construction (the standard for modern tires). 16 = the rim diameter in inches. This tire is designed to mount on a 16-inch wheel. The rim size must match exactly.
Check the sidewall of any tire on your vehicle — the size code is molded into the rubber, usually near the rim. You can also find the recommended tire size on a sticker inside the driver's door jamb, in your owner's manual, or in the glove box.
Yes, but the overall diameter should stay within about ±3% of the original to avoid speedometer error, clearance issues, and ABS problems. Use the Comparison Tool to check the exact diameter difference before buying.
Think of it like this: the first number is how wide the tire is (in mm), the second is how tall the sidewall is relative to that width (as a %), and the last number is what size wheel it fits (in inches). 205/55R16 = 205 mm wide, sidewall 55% of 205 mm, fits a 16-inch rim.
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