30 denotes the overall diameter of the tire (30”), and 3 1/2 denotes the sidewall area of the tire from the rim to the shoulder (3 1/2”). To ascertain the wheel diameter, double the second number and subtract from the first. Rim diameter: 30x3 1/2 = (3 1/2”+ 3 1/2”) – 30” = 23” inch rim.