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

Metric to inch tyre size conversion 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.

Taking metric to inch tyre size conversion as the case in point: 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.

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 — metric to inch tyre size conversion

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 metric to inch tyre size conversion.

The useful thing about metric to inch tyre size conversion 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 — metric to inch tyre size conversion

Taking metric to inch tyre size conversion as the case in point: 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.

Worked example — metric to inch tyre size conversion

On this page, metric to inch tyre size conversion 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 — metric to inch tyre size conversion

On this page, metric to inch tyre size conversion is the case in point: 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.

  • For metric to inch tyre size conversion, the point is this: Width: wider tyres change the sidewall height even when the aspect ratio stays the same, because the ratio is a percentage of the width.
  • For metric to inch tyre size conversion, the point is this: 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.
  • Rim diameter: each extra inch of rim adds 25.4 millimetres to the diameter before the sidewall is even counted. That is the point this page works through for metric to inch tyre size conversion.
  • On this page, metric to inch tyre size conversion is the case in point: Circumference: the distance the tyre covers in one revolution, which is what the speedometer and the gearing actually respond to.
  • Taking metric to inch tyre size conversion as the case in point: Revolutions per mile: fewer revolutions per mile means a taller tyre, and the difference shows up directly in the speedometer reading.
  • Taking metric to inch tyre size conversion as the case in point: 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 — metric to inch tyre size conversion

  • For metric to inch tyre size conversion, 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 metric to inch tyre size conversion 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.
  • For metric to inch tyre size conversion, the point is this: Treating the aspect ratio as a fixed height in millimetres instead of a percentage of the width.
  • Comparing two sizes by width alone and concluding they are the same diameter when their aspect ratios differ. That is the point this page works through for metric to inch tyre size conversion.

Step by step — metric to inch tyre size conversion

  1. Taking metric to inch tyre size conversion as 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. Taking metric to inch tyre size conversion as the case in point: Work out the sidewall height by multiplying the width by the aspect ratio and dividing by 100.
  3. On this page, metric to inch tyre size conversion is the case in point: Convert the rim diameter to millimetres by multiplying the inches by 25.4.
  4. Taking metric to inch tyre size conversion as the case in point: Add the rim diameter in millimetres to twice the sidewall height to get the overall diameter.
  5. On this page, metric to inch tyre size conversion 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, metric to inch tyre size conversion 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 — metric to inch tyre size conversion

Most problems with metric to inch tyre size conversion 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 — metric to inch tyre size conversion

Frequently asked questions

How many revolutions per mile does a tyre do?

For metric to inch tyre size conversion, 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 metric to inch tyre size conversion, 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.

For metric to inch tyre size conversion, why do two tyres with the same rim size have different diameters?

For metric to inch tyre size conversion, 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 metric to inch tyre size conversion, 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 metric to inch tyre size conversion, 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.