Key takeaways
- A code like 205/55 R16 91V tells you the width, profile, rim size, load index and speed rating in one string.
- The first number is the width in millimetres; the second is the profile, the sidewall height as a percentage of the width.
- The R means radial, the next number is the wheel rim diameter in inches, and the final figures are load and speed.
- Always match the size on your existing tyre or the door placard when buying replacements.
The string of numbers on your tyre sidewall looks like a code, and it is one. It tells you everything you need to buy the right replacement: the size, how much weight it can carry and how fast it is rated to go. Once you can read it, choosing tyres and checking you have the correct ones fitted becomes simple. Here is what each part means.
What do the numbers on a tyre mean?
The numbers on a tyre describe its size and ratings. A typical marking such as 205/55 R16 91V gives, in order, the width in millimetres, the profile as a percentage, the construction and rim diameter, the load index and the speed rating. Read together they define exactly which tyre fits your car and what it can safely handle.
Width and profile (the 205/55)
The first number is the tyre's width in millimetres across the tread — so 205 means 205mm wide. The second number is the profile or aspect ratio: the height of the sidewall as a percentage of the width. A 55 means the sidewall is 55% of 205mm. A lower number means a shorter, sportier sidewall; a higher number means a taller one.
These two figures change how the car feels. A wider tyre or lower profile can sharpen handling but may firm up the ride, while a taller profile cushions bumps. When replacing, match the original figures unless you are deliberately changing to an approved alternative size.
Construction and rim size (the R16)
The letter R stands for radial construction, which almost all modern car tyres use. The number straight after it is the wheel rim diameter in inches — R16 means the tyre fits a 16-inch wheel. This figure must match your wheels exactly; a tyre made for a different rim size simply will not fit.
Load index and speed rating (the 91V)
The last two markings are the load index and the speed rating. The number (91) is a code for the maximum weight the tyre can carry, and the letter (V) is the maximum speed it is rated for. You should fit tyres that at least match the load and speed ratings specified for your vehicle, never lower.
| Speed symbol | Max speed |
|---|---|
| T | 118 mph (190 km/h) |
| H | 130 mph (210 km/h) |
| V | 149 mph (240 km/h) |
| W | 168 mph (270 km/h) |
| Y | 186 mph (300 km/h) |
The speed rating is not an invitation to drive that fast — UK limits still apply. It reflects the tyre's design and heat tolerance, and fitting a lower-rated tyre than your car requires can affect handling and may invalidate insurance.
Why must all four match correctly?
The law requires tyres of compatible structure and size across an axle, and fitting mismatched tyres can be an MOT failure. Mixing radial and older cross-ply types on the same axle is not allowed, and large differences in size, tread pattern or wear between the two tyres on an axle can upset braking and handling, especially in the wet.
It is fine to run different brands front and rear if the sizes and ratings are right, but most fitters advise matching tyres in pairs across an axle so grip is balanced left to right. When in doubt, replacing two together keeps the car predictable under braking.
What are the other markings?
Beyond the size code, the sidewall carries other useful information. Most important is the DOT date code, a four-digit number showing the week and year the tyre was made — for example 2523 means the 25th week of 2023. You may also see M+S or a snowflake symbol for winter capability, and the maximum pressure and load.
Buying the right tyres
When you replace tyres, match the size and ratings on your current tyre or the door placard, and you cannot go wrong. Knowing how to read the code also helps with related checks — see our guides on checking your tyre pressure and how old is too old using the DOT code. When you are ready, Fast Tyre can supply and fit the correct tyres at your home or roadside through our mobile tyre fitting service, so you never have to guess at a counter.
Frequently asked questions
It is a tyre size code: 205 is the width in millimetres, 55 is the profile (sidewall height as a percentage of width), R means radial, 16 is the wheel rim diameter in inches, 91 is the load index for maximum weight, and V is the maximum speed rating.
It is moulded into the sidewall of your existing tyres and also listed on the placard inside the driver's door shut, on the fuel flap, or in the handbook. Matching either source ensures you buy the correct size and ratings for your car.
You should normally match the original size. Some approved alternative sizes exist, but the rim diameter must always match your wheels, and any change must keep the correct load and speed ratings. If unsure, stick to the size on the door placard or your current tyres.
The letter shows the maximum speed the tyre is designed to handle, such as V for 149 mph. UK speed limits still apply, but you must fit at least the rating your vehicle requires. A lower rating can affect handling and may invalidate your insurance.
The load index is a code for the maximum weight each tyre can safely carry at full pressure. A higher number means a higher capacity. Always fit tyres that meet or exceed the load index specified for your vehicle, which matters most on vans and loaded cars.
Look for the four-digit DOT date code on the sidewall. The first two digits are the week and the last two the year of manufacture, so 2523 means the 25th week of 2023. Most makers advise replacing tyres at around 10 years regardless of tread.