Every tyre carries its size moulded into the sidewall, written as a short code such as 205/55 R16 91V. It looks cryptic, but it always reads in the same order, and once each part is clear, matching the right replacement takes seconds rather than guesswork.
The size, read left to right
Taking 205/55 R16 91V as the example, each section has a fixed job:
| Part | Example | What it means |
|---|---|---|
| Width | 205 | Tread width in millimetres |
| Profile | 55 | Sidewall height as a percentage of the width |
| Construction | R | Radial construction |
| Wheel diameter | 16 | Fits a 16-inch wheel |
| Load index | 91 | Maximum weight per tyre (615kg) |
| Speed rating | V | Maximum rated speed (149mph) |
The first three numbers and the letter describe the tyre's physical size and how it is built. The last two, the load index and speed rating, describe its limits. Read together, this tyre is 205mm wide, has a sidewall 55% of that width, is of radial construction, fits a 16-inch wheel, can carry up to 615kg and is rated to 149mph.
Width: the first number
The first three-digit number is the section width in millimetres, measured across the widest point of the tyre. A 205 is 205mm wide. A wider tyre puts more rubber on the road, which can help grip, but it must still fit the wheel and the car's wheel arches, so the width is not a free choice.
Profile: the number after the slash
The two-digit number after the slash is the profile, also called the aspect ratio. This is the part that catches people out, because it is not a measurement in millimetres; it is the sidewall height expressed as a percentage of the width. A profile of 55 means the sidewall is 55% as tall as the tyre is wide. The same figure therefore produces a taller sidewall on a wider tyre, which is why two tyres with the same "55" can look quite different.
Construction: the letter
The letter after the profile describes how the tyre is built. On almost every modern car it is R, for radial, where the internal plies run across the tyre at 90 degrees to the direction of travel. An occasional high-performance tyre shows ZR, and very old or specialist tyres may use a D for diagonal (bias-ply) construction, now rare on the road.
Wheel diameter: the next number
The number after the construction letter is the diameter of the wheel the tyre is designed to fit, measured in inches. A 16 fits a 16-inch wheel. This figure must match the wheel exactly, a tyre cannot be stretched or squeezed onto a wheel of a different diameter.
Load index and speed rating: the limits
The number and letter at the end set the safe operating limits. The load index (91 in the example) is a code for the maximum weight the tyre can carry, read from a standard load chart, 91 equals 615kg per tyre. The speed rating (V) is a letter for the maximum speed the tyre is built to sustain, again from a chart, with V meaning 149mph.
Both are matched by the car maker to the weight and performance of the car, so a replacement should never have a lower load index or speed rating than the original specification. Going higher is generally acceptable; going lower is not.
Where to check the right size for a car
The size on the current tyres is the quickest reference, but the definitive source is the placard inside the driver's door, which lists the car maker's recommended size, pressures, load index and speed rating. The handbook carries the same information, and in the UK most tyre retailers, Tyres.co.uk among them, can return the correct size from a registration number. Where a tyre on the car differs from the placard, the placard is the figure to trust.
From the workshop: the most common mix-up is reading the profile as a measurement. Drivers will ask for a "55 sidewall" expecting a fixed height, but on a 225 tyre that 55 is taller than on a 195. Always quote the full size, never just one number.
Common questions
How do I read my tyre size?+
Read the code on the sidewall from left to right. In 205/55 R16 91V, 205 is the width in millimetres, 55 is the profile as a percentage of that width, R is radial construction, 16 is the wheel diameter in inches, 91 is the load index and V is the speed rating.
Where is the tyre size on the tyre?+
It is moulded into the outer sidewall, usually in raised lettering. The same size, as recommended by the car maker, is also on the placard inside the driver's door and in the handbook.
Which part of the size matters most when buying tyres?+
All of it should match, but the load index and speed rating must never be lower than the car maker specifies. The width, profile and wheel size must match the wheel, and the car's recommended size is the safe baseline.
Do all four tyres need to be the same size?+
Most cars use the same size on all four wheels, shown on the door placard. Some performance cars run a wider size on the rear, called a staggered fitment. The placard or handbook confirms what a specific car needs.