The two digit number tells us the aspect ratio or tire profile measurement. 55 indicates that the sidewall distance from the wheel rim to the outside of the tread, is 55% of the section width. R tells us that the tire has Radial construction. Over 98% of all tires sold today are radial tires where the internal body plies of the tire radiate outward from the center. 16 tells us that the tire should be matched to a 16-inch diameter wheel.