Tire size codes explained (205/55R16)
A tire code reads left to right: width in mm, sidewall height as a percentage of width, construction letter, rim diameter in inches, then load index and speed letter. Example: 205/55R16 91V.
| Code segment | Example | Meaning |
|---|---|---|
| 205 | 205 | Section width: tire is 205 mm wide at its widest point |
| /55 | 55 | Aspect ratio: sidewall height is 55% of width (≈113 mm) |
| R | R | Construction: R = radial (B = bias-belted, D = diagonal) |
| 16 | 16 | Rim diameter: fits a 16-inch wheel |
| 91 | 91 | Load index: 91 = 615 kg max load per tire |
| V | V | Speed rating: V = rated to 240 km/h (149 mph) |
| M+S / 3PMSF | M+S | Mud+Snow marking; 3PMSF snowflake = tested winter tire |
Notes
- Total tire diameter ≈ rim + 2 × sidewall: 16 in + 2 × 113 mm ≈ 632 mm for the example.
- Load index and speed rating must meet or exceed the placard values on the driver's door jamb.
- Winter tires may have a lower speed rating than the OEM tire — it's legal in most countries with a sticker reminder.
Reading a sidewall left to right
A tire code such as 205/55R16 91V is read in order, and each segment answers a different question. The first number, 205, is the section width in millimetres — the tire's widest point, measured across the sidewalls. The number after the slash, 55, is the aspect ratio: the sidewall height as a percentage of the width. The letter R means radial construction, still the overwhelming standard. The 16 is the rim diameter in inches that the tire fits. Finally, 91 is the load index and V is the speed rating. Reading it as one label rather than five facts is where most confusion starts.
From aspect ratio to physical size
The aspect ratio is the least intuitive part, because it is a percentage rather than a measurement. For 205/55, the sidewall height is 55% of 205 mm, which is about 113 mm. That gives the overall diameter: rim diameter plus two sidewall heights. In the example, 16 inches (about 406 mm) plus 2 × 113 mm is roughly 632 mm, or a little under 25 inches. Two tires can share a rim size and still be very different overall — a 205/55R16 and a 205/45R16 fit the same wheel but the second is noticeably shorter and will alter your speedometer reading, gearing and ground clearance.
Load index and speed rating
The load index is a code, not a weight: 91 corresponds to 615 kg per tire at maximum pressure, so four tires support about 2,460 kg. Higher indices carry more. The speed rating is a letter indicating the maximum sustained speed the tire is certified for — V is up to 240 km/h, H up to 210 km/h, T up to 190 km/h. Both must meet or exceed the values on the vehicle's placard, which is usually inside the driver's door jamb or on the fuel filler flap. Fitting a tire with a lower load index or speed rating than specified is not only risky, it can be illegal and may void insurance.
Changing sizes without breaking the car
Owners often want a wider or lower-profile tire, and it is possible within limits. The governing constraint is overall diameter: it should stay within about 3% of the original so that the speedometer, odometer, gearing and ABS calibration keep working as designed. Going wider usually means dropping the aspect ratio to keep the diameter close — so a 205/55R16 might become a 225/50R16. Width also has practical limits: a tire wider than the rim can safely hold will wear unevenly and handle poorly, and a wider tire may rub on the suspension or wheel arch. When in doubt, follow the sizes the manufacturer lists for your wheel.
Beyond the size: what else is on the sidewall
The size code is only the beginning. A sidewall also carries the DOT production code, whose last four digits are the week and year of manufacture — a tire older than about six years should be inspected closely even if the tread looks fine. The M+S or three-peak-mountain-snowflake marking tells you whether it qualifies as a mud-and-snow or true winter tire. Maximum inflation pressure, a maximum load warning, and sometimes the country of manufacture and an E-mark for European approval also appear. For winter tires in particular, the speed rating printed may be lower than the car's original, which many countries permit with the correct reminder sticker in the cabin.
Frequently asked questions
- What does 205/55R16 mean?
- A radial tire 205 mm wide, with sidewall height 55% of that width, mounted on a 16-inch rim. Add the load/speed (e.g. 91V) for the full code.
- Can I change the tire aspect ratio?
- Within limits — the overall diameter must stay close (usually ±3%) so speedometer, gearing and clearance work as designed.
- What does the load index 91 mean?
- Each tire can carry up to 615 kg at maximum pressure; four tires support 2460 kg total.