Tyre Size Conversion Chart: How It Works, What to Check, and Common Mistakes to Avoid

There is a lot of confident advice about tyre size conversion chart and much of it is wrong in the details, usually because it leaves out the conditions the answer depends on. This page works through the question properly: what it is, why it matters, how to do or check it, and the mistakes that catch people out.

For tyre size conversion chart, the point is this: Below you will find the explanation, a worked example with the assumptions stated, a table where one helps, and the factors that push the result up or down. Read the whole page before you act, because the step that matters most is often the one people skip.

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 conversion chart

Comparisons between sizes are where the maths earns its keep. Two tyres that look similar on paper can differ by several millimetres in diameter, and a change that looks small in the size code can be a change of several percent in diameter. Working the numbers for both sizes, side by side, shows the difference in millimetres and as a percentage, and from the percentage you can work out the speedometer error at any speed. That is the whole point of the calculation: to replace a guess with a number. That is the point this page works through for tyre size conversion chart.

What makes tyre size conversion chart worth understanding properly is that a small error at the start — a wrong figure, a skipped check — grows into a large error by the end, and the cost of that error is paid in tyres, fuel or safety.

How it works in practice — tyre size conversion chart

Tyre sizes are written in a mix of millimetres, percentages and inches, so working out a tyre's real diameter takes a small calculation rather than a glance. The formula is straightforward: take the rim diameter in inches and multiply by 25.4 to get millimetres, then add twice the sidewall height, where the sidewall height is the width in millimetres multiplied by the aspect ratio divided by 100. The result is the overall diameter in millimetres, and from there the circumference is pi times the diameter and the revolutions per mile follow from the circumference. That is the point this page works through for tyre size conversion chart.

Worked example — tyre size conversion chart

On this page, tyre size conversion chart is the case in point: Take a driver covering 12,000 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 conversion chart

The reason the diameter matters is that everything else follows from it. A taller tyre covers more ground per revolution, so the speedometer reads slow and the gearing effectively gets longer; a shorter tyre does the opposite. The commonly used guideline is to stay within about 3 percent of the original diameter when changing sizes, because beyond that the speedometer error, the gearing change and the clearance change all start to matter. That 3 percent figure is a guideline, not a safety approval, and it does not replace the placard check. That is the point this page works through for tyre size conversion chart.

  • Width: wider tyres change the sidewall height even when the aspect ratio stays the same, because the ratio is a percentage of the width. That is the point this page works through for tyre size conversion chart.
  • On this page, tyre size conversion chart is the case in point: Aspect ratio: a small change in the ratio moves the diameter by twice the change in sidewall height, so it matters more than it looks.
  • For tyre size conversion chart, the point is this: Rim diameter: each extra inch of rim adds 25.4 millimetres to the diameter before the sidewall is even counted.
  • For tyre size conversion chart, the point is this: Circumference: the distance the tyre covers in one revolution, which is what the speedometer and the gearing actually respond to.
  • Revolutions per mile: fewer revolutions per mile means a taller tyre, and the difference shows up directly in the speedometer reading. That is the point this page works through for tyre size conversion chart.
  • For tyre size conversion chart, the point is this: Wear: a worn tyre is a few millimetres smaller in diameter than a new one, so real-world diameters drift slightly from the nominal figures.

Common mistakes — tyre size conversion chart

  • For tyre size conversion chart, the point is this: Adding the sidewall height once instead of twice, forgetting that there is a sidewall at the top and at the bottom of the tyre.
  • Taking tyre size conversion chart as the case in point: Mixing inches and millimetres in the same step, for example adding the rim diameter in inches to a sidewall in millimetres.
  • Treating the aspect ratio as a fixed height in millimetres instead of a percentage of the width. That is the point this page works through for tyre size conversion chart.
  • For tyre size conversion chart, the point is this: Comparing two sizes by width alone and concluding they are the same diameter when their aspect ratios differ.

Step by step — tyre size conversion chart

  1. On this page, tyre size conversion chart is the case in point: Write down the size code exactly, for example 205/55R16, and split it into width, aspect ratio and rim diameter.
  2. Work out the sidewall height by multiplying the width by the aspect ratio and dividing by 100. That is the point this page works through for tyre size conversion chart.
  3. On this page, tyre size conversion chart is the case in point: Convert the rim diameter to millimetres by multiplying the inches by 25.4.
  4. For tyre size conversion chart, the point is this: Add the rim diameter in millimetres to twice the sidewall height to get the overall diameter.
  5. On this page, tyre size conversion chart is the case in point: Multiply the diameter by pi to get the circumference, and divide 63,360 by the circumference in inches for revolutions per mile.
  6. On this page, tyre size conversion chart is the case in point: Repeat the same steps for the second size and compare the two diameters as a difference in millimetres and as a percentage.

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

Most problems with tyre size conversion chart 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 conversion chart

Frequently asked questions

How many revolutions per mile does a tyre do?

For tyre size conversion chart, it depends on the circumference: divide 63,360, the number of inches in a mile, by the circumference in inches. A typical car tyre does roughly 750 to 850 revolutions per mile, with taller tyres doing fewer and shorter tyres doing more. The exact figure comes from the size chart for the size in question.

Can I compare two tyre sizes with a calculator?

For tyre size conversion chart, yes, and it is the reliable way to do it. Enter both sizes and the calculator works out each diameter, the difference in millimetres and as a percentage, and the speedometer effect at common speeds. Always check the result against the placard and the plus-or-minus 3 percent guideline before deciding anything.

Why do two tyres with the same rim size have different diameters for tyre size conversion chart?

For tyre size conversion chart, because the diameter also depends on the width and the aspect ratio. A 205/55R16 and a 225/45R17 sit on different rims, but even on the same rim a wider tyre or a higher aspect ratio gives a taller sidewall and a bigger diameter. The size code has three numbers for a reason, and all three decide the diameter.

How do I calculate a tyre's diameter?

For tyre size conversion chart, multiply the rim diameter in inches by 25.4 to get millimetres, then add twice the sidewall height. The sidewall height is the width in millimetres times the aspect ratio divided by 100. For a 205/55R16 that gives a rim of 406.4 millimetres plus two sidewalls of about 112.8 millimetres, for an overall diameter of about 631.9 millimetres. These are nominal new-tyre figures.

What is tyre circumference and why does it matter?

For tyre size conversion chart, circumference is the distance a tyre covers in one full revolution, worked out as pi times the diameter. It matters because the speedometer, the gearing and the odometer all respond to how far the tyre travels per revolution, so a change in circumference changes all three. A taller tyre covers more ground per turn and the speedometer reads slow.