What is 285/70R17 in inches?
Answer
32.71 in tall
285/70R17 is 32.71 in in overall diameter and 11.22 in in section width, on a 17 in wheel. Its sidewall is 7.85 in tall.
The working
- Overall diameter
- 32.71 in
- Section width
- 11.22 in
- Sidewall height
- 7.85 in
- Circumference
- 102.8 in
- Revolutions per mile
- 616.6
- Nearest whole-inch call size
- 32.5 in
How the number is arrived at
Where 32.71 in comes from: the aspect ratio makes the sidewall 199.5 mm, which is 7.85 in, and a diameter contains two of them — top and bottom — on top of the 17 in of wheel. Two sidewalls is 15.71 in, and 17 plus 15.71 is 32.71 in. (The full anatomy of a size code walks through each field if you want the general rule rather than this one answer.)
Section width converts directly: 285 mm is 11.22 in — the widest point of the inflated casing, which is not the tread and is usually a little wider than it.
The answer people are usually really after
Whether it is a 33. It is 0.29 in short of one — a genuine 33x12.50R17 measures 33.00 in, because in imperial notation the first number is the diameter. That is 0.89%, which is inside the tolerance of most speedometers and outside the tolerance of some fender lips.
The gap in width is larger and gets less attention: 11.22 in against 12.50 in, a difference of 1.28 in.
Where the geometry stops matching the tire
This figure is computed from the size code, which is a specification of the casing. A real tire may measure a few tenths either way depending on the manufacturer, the tread pattern and the rim it is mounted on. Aggressive patterns commonly measure under their stated size. If a tenth of an inch decides your answer, measure the tire rather than the code.
The 11.22 in figure moves the most, because rim width changes it: the same casing stretched over a wider wheel measures wider and sits fractionally lower. Two people can measure the same size, get different numbers, and both be reporting honestly.
The other numbers that follow
Circumference is 102.8 in, which gives 616.6 revolutions per mile. That figure is what every downstream consequence is derived from: fit this size in place of something shorter and the speedometer under-reads by exactly the proportion the circumference grew.