What is 245/75R16 in inches?

Half an inch short of the name it is given

30.47 in. Round that to the nearest whole inch and you get 30, not 31, yet this size is habitually listed as a 31-inch tire in fitment charts and conversation. It is one of the clearer cases of a nickname drifting upward, and it matters because the sizes people compare it against — genuine 31x10.50R15s at 31.00 in — really are what they claim.

Why the shortfall exists

75% of 245 mm gives 183.8 mm of sidewall, which is 7.23 in. Stack one at each end of a 16-inch rim and the total stops at 30.47 in. To arrive at a true 31 on a 16-inch rim, either the section has to grow or the aspect ratio does; there is no arrangement of a 245 and a 75-series that gets there.

The clean one-inch step

245/75R17 is the same section width and the same aspect ratio on a wheel one inch larger, and it measures 31.47 in — exactly 1.00 in taller. That is the tidiest demonstration available of what a rim size does on its own: identical rubber, identical proportions, one inch of extra diameter, all of it wheel. The speedometer change is 3.28%.

The other direction, and the tire under the truck

Widening instead of enlarging the rim, 265/75R16 reaches 31.65 in — 1.18 in taller and 0.79 in wider on the same 16-inch wheel. Between them, those two moves cover almost every way this size gets replaced. Load either pair into the comparison to see the difference stated in inches.

245/75R16 also survives in quantity as a full-size spare on trucks that wear something larger on the road, which is where a lot of people first meet the size and first find out that it does not match what is at the corners. At 661.9 revolutions per mile it counts faster than anything it is likely to be mounted alongside, which is worth knowing before a spare goes on for a long stretch.