Tire sizes are printed in a mix of units. Width is in millimetres, the sidewall is a percentage, and the rim is in inches. That is fine until you want to know how tall a tire actually is, or whether a set you found online will fit like the one you have now. The fix is to convert the code into one unit. Inches are the usual choice in the US, because lift kits, fender clearances and tire shop listings all use them.
This walkthrough uses 275/55R20 as the example. The same steps work for any metric size.
What the three numbers mean
In 275/55R20:
- 275 is the section width in millimetres, measured across the tire from sidewall to sidewall.
- 55 is the aspect ratio. The sidewall height is 55% of the width.
- R20 means radial construction and a 20-inch rim diameter.
The width and the ratio are the part that trips people up. A 55 does not mean 55 millimetres. It means 55% of 275 mm.
Step 1: Find the sidewall height
Multiply the width by the aspect ratio and divide by 100. For 275/55R20 that gives a sidewall of 151 mm, which is about 6.0 inches once you divide by 25.4. Do this in millimetres first and convert at the end. Rounding the sidewall to whole inches before the next step is the most common way the final diameter ends up half an inch off.
Step 2: Add two sidewalls to the rim
The tire has a sidewall above the rim and another below it, so the overall diameter is the rim diameter plus two sidewall heights. For this size the result is 31.9 in. The width converts directly: 275 mm is 10.8 in.
So a 275/55R20 is about 31.9 inches tall and 10.8 inches wide. These are nominal figures calculated from the size code. A real tire from a given brand can come out a little different, depending on the model, the rim width it is mounted on and its inflation. Tires also grow a little when inflated and sag a little under load, so treat the number as a close estimate, not a measurement.
Step 3: Get circumference and revolutions per mile
Circumference is the diameter times pi, which comes to 100.2 in for this tire. A mile is 63,360 inches, so dividing by the circumference gives about 632 revolutions per mile.
This number matters because your speedometer and odometer count wheel rotations. If the new tire turns fewer times per mile than the old one, the car covers more ground per rotation and the speedometer reads low. The actual speed is the displayed speed multiplied by the new diameter and divided by the old diameter, and the odometer is off by the same ratio.
Using the numbers to compare two sizes
Take a second size, 265/70R17, which is common on trucks and SUVs. Run the same three steps and it comes out at roughly 31.6 inches tall. The two tires look nothing alike on paper, with a different rim, a different ratio and a different width. But 31.9 in against 31.6 in is a difference of about 1%. The 275/55R20 is slightly taller and noticeably wider, with a much shorter sidewall, which means less cushion over potholes and a sharper look in the wheel well. Converting to inches is what makes all of that visible, because the codes themselves hide it.
A common rule of thumb is to keep the overall diameter within about 3% of the original size. Past that, the speedometer error gets large enough to matter, and a tire that is taller than the original means you are going faster than the dash says. Gearing, the calibration of ABS and traction control, and clearance in the wheel well are the other things that change with diameter, so a size that is close to the original is the low-risk choice.
Common mistakes
- Treating the rim size as the tire size. A 20-inch rim does not make a 20-inch tire. The 275/55R20 above is nearly 32 inches tall.
- Comparing only the width. A 285 is not necessarily a bigger tire than a 265 if the aspect ratio drops with it. Check the diameter.
- Ignoring the rim width. A tire has a range of rim widths it is approved for. The nominal width in the code assumes a specific measuring rim.
- Assuming fit. The conversion tells you the size. It does not tell you whether the tire clears your suspension and fender at full lock and full compression. Measure on the vehicle or ask the shop.
Doing it faster
The arithmetic is short, but doing it for several candidate sizes gets tedious. A 275/55R20 in inches page gives the diameter, width, sidewall and revolutions per mile for that size directly, and the same site has a converter and a two-size comparison for checking speedometer error.
Whichever way you do it, the order is the same: convert the code to diameter and width, compare against the tire you have now, and keep the diameter change small unless you have a reason to change it.
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