Tyre Size Explained: A Practical Guide With a Worked Example and Clear Next Steps

Tyre size explained is one of those questions that sounds simple until you try to answer it for your own car. The answer depends on the car, the conditions and how you drive, so there is no single figure that fits every driver. What this page does instead is give you the method, so you can work out your own answer and know exactly what it means.

The sections below take the question in order: the explanation first, then the step-by-step method, then the things that can go wrong and the signs that it is time to see a professional. Every number that depends on a place or a rule states the jurisdiction it applies to, because limits change and a figure that is right in one country can be wrong in another. That is the point this page works through for tyre size explained.

This is a calculation, not a safety approval. Always check your vehicle's door-jamb placard, load index and speed rating, and confirm fitment with a tyre professional before fitting a different tyre size.

What you need to know first — tyre size explained

Taking tyre size explained as the case in point: A tyre size code looks like a random string of numbers and letters until you know what each part means, and then it reads like a short specification. The first number is the width of the tyre in millimetres, measured across the tread from one sidewall to the other. The second number, after the slash, is the aspect ratio: the height of the sidewall expressed as a percentage of the width, so a 55 means the sidewall is 55 percent as tall as the tyre is wide. The letter that follows is usually R, which stands for radial construction, and the final number is the diameter of the wheel rim in inches that the tyre fits.

The useful thing about tyre size explained is that it can be checked with tools most drivers already own: a gauge, a tread-depth measure and the handbook for the car.

How it works in practice — tyre size explained

On this page, tyre size explained is the case in point: Once you can read the code, the rest of the sidewall starts to make sense too. After the size you will usually find a load index number and a speed rating letter, which together say how much weight the tyre can carry and how fast it is rated to run. Those two markings matter as much as the size itself, because a tyre that is the right size but the wrong load index is not the right tyre for the car. The sidewall also carries the brand, the model name, the manufacturing date code and, on many tyres, a tread-wear indicator marking.

Worked example — tyre size explained

For tyre size explained, the point is this: Take a driver covering 14,000 miles a year as an example figure. The method on this page works the same at any mileage, so the example is there to show the arithmetic, not to state a fact about your car. Start with the measurement or the figure you can check yourself — a gauge reading, a tread-depth reading, or the figure on the placard — and write it down before you do anything else. Then apply the method step by step, checking each result against the one before it, because an error early in the chain is the one that is hardest to spot later. If the result surprises you, check the inputs first: a pressure read on warm tyres, a tread depth measured in one spot only, or a figure taken from the fitted tyre instead of the placard will each give a wrong answer that looks plausible. That is why the worked example on this page states its assumptions openly, and why you should do the same with your own figures.

What changes the answer — tyre size explained

The most reliable way to find the size your car needs is the door-jamb placard, the sticker on the driver's door frame that lists the manufacturer's recommended size, pressure and load rating. The owner's manual lists the same information, and the current tyre's sidewall shows what is fitted now — but the fitted tyre is not always the size the manufacturer specified, so the placard wins when the two disagree. If you are unsure, a tyre professional can confirm the size from the registration or the vehicle identification number. That is the point this page works through for tyre size explained.

  • For tyre size explained, the point is this: Width: a wider tyre spreads the load over a bigger contact patch, which can improve grip but can also increase fuel use and road noise.
  • For tyre size explained, the point is this: Aspect ratio: a lower number means a shorter sidewall, which usually sharpens steering response but gives a firmer ride.
  • Rim diameter: the tyre must match the wheel it sits on; a 16-inch tyre does not fit a 17-inch wheel, however close the other numbers look. That is the point this page works through for tyre size explained.
  • Taking tyre size explained as the case in point: Load index: the number after the size must meet or exceed the figure on the placard, because it is the tyre's weight limit.
  • Speed rating: the letter after the load index is the maximum speed the tyre is rated for, and it should match the placard figure. That is the point this page works through for tyre size explained.
  • On this page, tyre size explained is the case in point: Construction letter: R means radial, which is the construction on almost every modern car tyre; anything else is a specialist case.

Common mistakes — tyre size explained

  • On this page, tyre size explained is the case in point: Reading the aspect ratio as a millimetre measurement instead of a percentage of the width, which makes the sidewall height look far bigger than it is.
  • On this page, tyre size explained is the case in point: Assuming the size on the fitted tyre is the size the manufacturer specified, when a previous owner may have changed it.
  • Ignoring the load index and speed rating and choosing on size alone, which leaves the tyre under-rated for the car. That is the point this page works through for tyre size explained.
  • On this page, tyre size explained is the case in point: Mixing up the rim diameter in inches with the tyre width in millimetres, two numbers that use different units on the same code.

Step by step — tyre size explained

  1. Open the driver's door and find the placard on the door frame or the inside of the fuel flap. That is the point this page works through for tyre size explained.
  2. On this page, tyre size explained is the case in point: Read the size listed there and write it down exactly, including the load index and speed rating.
  3. On this page, tyre size explained is the case in point: Walk to the car and read the size on the sidewall of a fitted tyre, to compare it with the placard.
  4. For tyre size explained, the point is this: Break the code into its parts: width in millimetres, aspect ratio as a percentage, R for radial, rim in inches.
  5. For tyre size explained, the point is this: Note the load index number and the speed rating letter that follow the size code.
  6. If the placard and the fitted tyre disagree, ask a tyre professional to confirm which size the car should run. That is the point this page works through for tyre size explained.

What can go wrong, and when to see a professional — tyre size explained

Most problems with tyre size explained announce themselves early if you know what to look for: uneven wear, a vibration that was not there before, a warning light, a noise that changes with speed, or a reading that does not match the placard. None of these is a reason to panic, and all of them are reasons to check the basics first — pressures with a gauge when the tyres are cold, tread depth across the whole tread, and the figures on the placard rather than the ones on the fitted tyre. If the basics are right and the symptom is still there, or if you find a bulge, a deep cut, a cracked sidewall or a tyre that is losing pressure quickly, that is the point to see a tyre professional rather than to keep driving and hoping. A professional can see the inside of a tyre, measure the alignment angles and test the sensors, and those are the checks that cannot be done from the driveway. The cost of an inspection is small beside the cost of a tyre worn out early or a problem found too late.

Related guides — tyre size explained

Frequently asked questions

What does the R in a tyre size stand for?

For tyre size explained, r stands for radial construction, which is how almost every modern car tyre is built, with the cord plies running radially across the tyre. You may occasionally see D for diagonal or B for belted construction on older or specialist tyres, but for a normal car the letter will be R.

Does the load index matter if the size is right?

For tyre size explained, yes. The load index is the maximum weight each tyre can carry, and a tyre with too low a load index is under-rated for the car even if the size matches. Always match or exceed the load index on the placard, and check the speed rating at the same time.

Where do I find my tyre size?

For tyre size explained, the door-jamb placard on the driver's door frame is the first place to look, and it lists the size the manufacturer specified for your car. The owner's manual lists the same information, and the sidewall of any fitted tyre shows what is on the car now. When the placard and the fitted tyre disagree, trust the placard and confirm with a tyre professional before buying.

What does the first number in a tyre size mean?

For tyre size explained, the first number is the width of the tyre in millimetres, measured across the tread. A 205 tyre is 205 millimetres wide, a 225 tyre is 225 millimetres wide, and so on. Width alone does not tell you the diameter, because the sidewall height depends on the aspect ratio as well.

What does the number after the slash mean?

For tyre size explained, it is the aspect ratio: the sidewall height as a percentage of the tyre width. On a 205/55 tyre the sidewall is 55 percent of 205 millimetres, which works out to about 113 millimetres. A lower aspect ratio means a shorter, stiffer sidewall; a higher one means a taller, more flexible sidewall.