Once you want to buy a new press-on solid tire, but maybe the tire has been worn out to the point you can not read the tire size, then how can you know the tire size without any mark on the old tire? Press-on solid tires have 3 measurements - tire outside diameter, tire width, and tire inside diameter or the size of the hub which the tire fits on. These measurements are expressed in the tire size; for example, a 18x8x18 1/8, it is 18 inches tall, 8 inches wide and has an inside diameter of 18 1/8 inches.
See the sample picture below
Tire outside diameter “18”
Tire width “8”
Tire inside diameter “18 1/8 ”
Noted, when measuring the inside diameter (18 1/8) measure from the inside of the band and not from where the rubber meets the band.