How to Read Tyre Size Markings: The Complete UK Guide

Look at the sidewall of any tyre and you’ll see a string of numbers and letters that looks something like this: 205/55 R16 91V. To most drivers this is meaningless. To a tyre fitter, it tells them everything they need to know. This guide explains every part of the code in plain English.

The Quick Answer: What Does 205/55 R16 91V Mean?

Part 1: Tyre Width (the first number)

The first number — 205 in our example — is the tyre’s width measured in millimetres across the tread, from sidewall to sidewall. Common UK widths range from around 155mm on small city cars to 285mm or wider on large SUVs and performance vehicles.

A wider tyre generally gives more grip in dry conditions but can perform worse in the wet and uses more fuel. A narrower tyre cuts through standing water more effectively, which is why some winter tyres are deliberately narrower than the summer equivalent.

Part 2: Aspect Ratio (the number after the slash)

The aspect ratio — 55 here — is the height of the tyre’s sidewall expressed as a percentage of the tyre’s width. So a 205/55 tyre has a sidewall height of 55% of 205mm, which works out to approximately 113mm.

Lower aspect ratios (35, 40, 45) mean shorter sidewalls, which you typically see on sporty or performance cars. They look sleek but give a firmer, less forgiving ride and are more vulnerable to pothole damage. Higher aspect ratios (65, 70, 75) mean taller sidewalls, which absorb bumps better — common on SUVs, vans and older cars.

Part 3: Construction Type (the letter R)

The R stands for Radial, which refers to how the internal cords of the tyre are arranged — running radially from bead to bead, at 90 degrees to the direction of travel. Almost every tyre sold in the UK today is radial construction. You may occasionally see a D (diagonal or bias-ply) on older or specialist tyres, but it’s rare on modern passenger cars.

Part 4: Rim Diameter (the number after R)

This is the diameter of the wheel rim the tyre is designed to fit, measured in inches. The 16 in our example means the tyre fits a 16-inch rim. This must match your wheel exactly — a tyre marked R16 will not fit a 17-inch rim, and vice versa.

UK cars have moved towards larger rim sizes over the years. 15 and 16-inch rims were standard on family cars for decades; 17, 18 and 19-inch rims are now common, particularly on SUVs and premium vehicles. Larger rims look good but typically mean lower profile tyres, which are more expensive and more prone to damage on poor road surfaces.

Part 5: Load Index (the two-digit number)

The load index tells you the maximum weight each tyre can support when properly inflated. It’s not a weight figure itself — it’s a code that corresponds to a weight on a standardised table. Here are the most common ones for UK passenger cars:

Always fit tyres with at least the same load index as specified in your vehicle handbook. Fitting a lower load index tyre than required is both unsafe and may invalidate your insurance.

Part 6: Speed Rating (the letter at the end)

The speed rating letter indicates the maximum sustained speed the tyre is designed to handle. Again, this is a code not an actual speed figure:

As with load index, you should always match or exceed your vehicle’s specified speed rating. The speed rating is a safety specification, not a performance upgrade — fitting a T-rated tyre on a car that specifies V-rated is dangerous and potentially illegal.

Where to Find Your Correct Tyre Size

Not sure what size your car needs? You have three options:

1. Check the tyre sidewall on your existing tyres
2. Look inside your driver’s door jamb — most cars have a sticker with the recommended tyre size and pressure
3. Check your vehicle handbook — usually lists front and rear sizes separately

Note that some cars are designed to run different size tyres on the front and rear axles (staggered fitment), particularly performance and sports cars. Always check front and rear separately if you’re unsure.

Should You Ever Change Your Tyre Size?

In short: not without doing your research. Changing tyre size affects your speedometer accuracy, fuel consumption, handling, and the load-carrying capacity of your vehicle. Some changes are within acceptable tolerances; others can affect your MOT, insurance, or safety.

If you’re considering upsizing — fitting larger wheels with lower profile tyres — make sure the overall rolling diameter stays as close as possible to the original specification. A tyre calculator can help you compare the rolling diameter of different size combinations.

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