A "31 inch" tyre is about 31 inches tall overall. In metric sizes that means different widths and profiles on each rim size. Here are the sizes that come out at about 31 inches (tires sold as 31s), with exact heights.
Sidewall height = width × profile ÷ 100, in millimetres. Overall height = rim diameter in inches + two sidewalls ÷ 25.4. For example, 285/70R17: 285 × 0.70 = 199.5 mm per sidewall, so 17 + 2 × 7.85 = 32.7 inches, sold as a 33. These are nominal sizes; real tyres differ by model, tread depth and pressure.
There isn't one size: it depends on the rim. On 16-inch rims, 245/75 R16 is 30.5 inches tall; On 17-inch rims, 245/70 R17 is 30.5 inches tall; On 18-inch rims, 265/60 R18 is 30.5 inches tall.
Is 245/75 R16 a 31 inch tyre?+
Close. Its nominal height is 30.5 inches (774 mm), so it's usually sold and talked about as a 31. Real tyres vary by model and pressure, so check the maker's published diameter.
How wide is a 31 inch tyre?+
Width varies by size. The metric sizes here run from 9.3 to 12.4 inches wide; the second number on a flotation size like 31x12.50 is the width in inches.
Will 31 inch tyres fit my car or truck?+
That depends on wheel well space, wheel offset, suspension and steering lock. Compare the new height with your stock size; a bigger change usually needs more clearance, a lift or trimming, and moves the speedometer.