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Reference

How to read a tire size

The whole code

285/70R17 is four readings in one string: 285 millimeters across the section, a sidewall 70% of that width, radial construction, and a 17 inch wheel. Put through the formula that is 32.71 in tall overall.

The four parts of the size code 285/70R17The code 285/70R17 set large, with a leader line from each of its four parts down to a label: 285 is the section width in millimeters, 70 is the aspect ratio as a percentage of that width, R is radial construction, and 17 is the wheel diameter in inches. All four readings are written out in the steps on this page.285/70R17section widthin millimeterswheel diameterin inchesaspect ratio% of that widthradialconstruction
Every part of the code, with what it names.

The same four readings, on the tire itself

Each number, and what it names. The five steps below work through them in order.

285/70R17 drawn head-on, with each reading namedA 285/70R17 tire section drawn head-on to scale on its wheel, with four labeled readings beside it. Reading down: 285 mm is the section width, measured at its widest point; 199.5 mm is the sidewall height, which is 70 percent of the section width; 17 in is the wheel it is built for; and 32.71 in is the overall diameter, the wheel plus two sidewalls. Every value here is written out in the steps on this page.285 mmsection width, at itswidest point199.5 mmsidewall height —70% of 28517 inthe wheel it isbuilt for32.71 inoverall diameter —wheel + two sidewalls
The tire seen head-on, cut through, at one scale. Every label carries the number and what the number names.

Five steps, on one worked example

The example is 285/70R17 throughout.

  1. Read the three-part code off the sidewall

    It is molded into the rubber, usually the largest string on the wall: 285/70R17.

  2. The first number is the section width, in millimeters

    285 mm, which is 11.22 in. It is measured at the widest point of the unloaded casing, not across the tread.

  3. The second number is the aspect ratio, as a percentage of that width

    70 means the sidewall is that percentage of 285 mm, so 199.5 mm — 7.85 in. It is a ratio, not a length, which is why the same number means something different on a narrow tire and on a wide one.

  4. The letter is construction; the last number is wheel diameter in inches

    R is radial. 17 is the wheel it is built for, and that one is exact — no conversion, no tolerance.

  5. Compute the overall diameter

    Wheel plus two sidewalls: 17 + (285 × 70 × 2) ÷ 2540 = 32.71 in.

What the code does not tell you

Load index, speed rating, load range and maximum inflation pressure are all stamped elsewhere on the wall, and none of them is derivable from these four readings. Neither is the rim width the size is approved for, or whether it clears your arches. This site computes dimensions and says so; anything beyond that belongs to the tire manufacturer's own fitment table.

Flotation sizes read differently

A code like 33x12.50R15 states the overall diameter and the section width in inches, in that order, with the wheel last. Nothing needs converting: the 33 is already the height.