31-inch tires
A 31-inch tire is about 31 inches tall. In metric, the closest sizes are 255/75R16 (31.1 in), 255/70R17 (31.1 in), 275/60R18 (31.0 in), 255/60R19 (31.0 in).
"31s" is shorthand for any tire close to 31 inches tall. Inch sizes say it outright; metric sizes get there from width and sidewall, so each rim size has its own set. The bars below are drawn to the same scale, so you can see which ones really are 31 inches.
31x inch sizes
Metric sizes about 31 inches tall
15-inch rims
17-inch rims
19-inch rims
21-inch rims
24-inch rims
31-inch tire questions
What size is a 31-inch tire in metric?
It depends on the rim. On a 16-inch rim, 255/75R16 is 31.1 inches tall. On a 17-inch rim, 255/70R17 is 31.1 inches tall. On an 18-inch rim, 275/60R18 is 31.0 inches tall. On a 19-inch rim, 255/60R19 is 31.0 inches tall.
Is a 31x12.50 the same as a metric 31?
Not quite. 31x10.50R15 is a true 31 inches tall and 10.5 inches wide, while metric "31s" range from 30.5 to 31.5 inches and are usually narrower.
Will 31-inch tires change my speedometer?
Yes, if they are taller or shorter than the size you have now. Going from a 31.6-inch 265/70R17 to a 31-inch tire changes the reading by about 1.9%. Put your own size into the tire size calculator to see the exact change.