305/40R21

305 mm across the tread, a sidewall 40% of that — 122 mm — on a 21-inch rim. That puts the whole wheel at 30.6 inches tall.

It mounts on a rim between 8.5" and 11" wide, and the tread follows the rim: roughly 5 mm of section width for every half-inch, so about 293 mm across at the narrow end and 318 mm at the wide one. Written the way lift kits and wheel wells are, it is a 30.6×12.0R21 — tall by wide, in inches.

Same 21-inch rim, height within 3%. Past that the speedometer starts lying and things begin to rub.

Same height, different rim

Bigger rim, same overall height. Sharper steering, harsher ride, and far less rubber between your wheel and the next pothole.

If you fit something taller

A taller tire covers more ground per turn, so the needle reads low and the odometer counts short. Over ten thousand miles on the dial, the last column is what you really drove.