How to Read Tire Size: What 205/55 R16 Actually Means

Breaking the code into three parts

A metric tire size like 205/55 R16 packs three measurements into one string. The first number, 205, is the nominal section width in millimetres — the distance across the tread from sidewall to sidewall. The second number, 55, is the aspect ratio: it is not a length but a percentage. It tells you the sidewall height as a share of the width.

The letter R stands for radial construction, the standard for almost every passenger car built in the last few decades. The final number, 16, is the wheel rim diameter in inches — the one imperial holdout in an otherwise metric code. So R16 means the tire is designed to fit a 16-inch wheel, and nothing else will seat correctly.

Turning the code into an overall diameter

To get the sidewall height, multiply the width by the aspect ratio: 205 × 0.55 = 112.75 mm. Because a tire has a sidewall above and below the rim, you count that height twice. Add the rim diameter, converted from inches: 16 × 25.4 = 406.4 mm. The overall diameter is 406.4 + 2 × 112.75 = 631.9 mm, roughly 24.9 inches.

This overall diameter matters because it sets how far the car travels per wheel revolution, which is how the speedometer and odometer are calibrated. A tire that changes the diameter by more than about 3% will make those readings wrong and can upset ABS and traction control. Widen the tread or change the aspect ratio and you must adjust the other numbers to keep the diameter close.

The rest of the sidewall markings

After the size you will usually see a two- or three-digit load index and a letter speed rating, such as 91V. The 91 maps to a maximum load per tire (615 kg here), and V means the tire is rated up to 149 mph. Fitting a lower load index or slower speed rating than the vehicle calls for is unsafe, even if the size matches.

You will also find a DOT code whose last four digits are the week and year of manufacture — 3623 means the 36th week of 2023. Tires age even unused, so this date helps you avoid old stock. Reading the whole sidewall, not just the size, is what tells you a tire is genuinely a safe match for your car.

Frequently asked questions

What does the slash in 205/55 mean?

The slash separates the section width (205 mm) from the aspect ratio (55%). It is just a separator, not a division — 55 is the sidewall height as a percentage of the 205 mm width, giving 112.75 mm.

Can I fit a different tire size than the original?

Only if the replacement keeps the overall diameter within about 3% and matches or exceeds the load and speed ratings. A plus-size setup with a wider tread and lower aspect ratio can keep the diameter while changing the look, but always check clearance and the placard.

Why is the rim in inches but the rest in millimetres?

It is a historical mix. Wheel diameters were standardized in inches long before tire dimensions moved to metric, so the industry kept inches for the rim to stay compatible with existing wheels while widths and heights went metric.

Where do I find my correct tire size?

The factory-recommended size is on the placard inside the driver's door jamb and in the owner's manual, not just on the current tires — a previous owner may have fitted the wrong size. Always trust the placard over what happens to be on the car.