How tall is a 31 inch tire, really?
Answer
30.56–31.46 in
"31-inch" is a category, not a measurement. Of the sizes in this catalogue that people call 31s, the geometric diameters run from 30.56 in to 31.46 in — a spread of 0.90 in between tires sold under the same name.
The working
- 31x10.50R15
- 31.00 in tall, 10.50 in wide
- 31x10.50R16
- 31.00 in tall, 10.50 in wide
- 255/65R18
- 31.05 in tall, 10.04 in wide
- 255/70R17
- 31.06 in tall, 10.04 in wide
- 275/45R21
- 30.74 in tall, 10.83 in wide
- 265/70R16
- 30.61 in tall, 10.43 in wide
- 265/65R17
- 30.56 in tall, 10.43 in wide
- 285/60R18
- 31.46 in tall, 11.22 in wide
The label with the most drift in it
Flotation 31s are honest: 31x10.50R15 and 31x10.50R16 both measure 31.00 in. The trouble is everything else that gets called a 31, which runs from well under to well over:
- 245/75R16 — 30.47 in. Routinely listed as a 31 and more than half an inch short of one.
- 265/65R17 — 30.56 in. Also called a 31, also nearer 30.5.
- 265/70R16 — 30.61 in.
- 255/65R18 — 31.05 in and 255/70R17 — 31.06 in. These two genuinely are 31s.
- 285/60R18 — 31.46 in and 245/75R17 — 31.47 in. Half an inch over.
- 265/70R17 — 31.61 in. Called a 31 by some and a 32 by others.
An inch of spread under one name
From 30.47 in to 31.61 in is 1.14 in between the shortest and the tallest tire that people describe with the same two digits — more range than exists between a genuine 31 and a genuine 32. No other call size in common use is this vague.
The honest 31s only exist on small wheels
Flotation 31s are built on 15-inch and 16-inch rims and essentially nowhere else. There is no 31x10.50R17 or R18 in ordinary circulation, because by the time wheels reached that size the market had moved to metric codes. So a modern truck on 17-inch or larger wheels that wants "31s" has no exact option at all — only the metric approximations above, which is precisely how the label came loose in the first place.
Note also what the flotation version carries in sidewall: 8.00 in on a 31x10.50R15, against 6.78 in on a 265/65R17 of nearly the same height. The name says nothing about that either.
Why the 31 in particular
It is the oldest of the modern flotation call sizes and it survived into an era when metric codes replaced flotation ones on most vehicles. Sizes that arrived later inherited the nickname by rough resemblance rather than by measurement, and nothing ever tightened the usage.
Working around it
If a fitment claim, a lift kit chart or a second-hand listing says "31s", it has narrowed the diameter to a range of about an inch and nothing more. Only the size code resolves it. Take the two extremes from the list above and run them together: the answer arrives in hundredths of an inch rather than in nicknames.