sizes have letters in them, but they don't always have the same
letters. You're probably familiar with the "P" or the "LT" that
sometimes shows up at the beginning of the size, and the "R" right
before the wheel size, for example P245/40R18 or LT315/70R17. But
what does it mean when the tire says "P245/40ZR18"? For that, you
need to know a little bit...