You can find all the basic details about a tire by reading the information on the sidewall.
255 = Tire Width
Measured in millimeters from sidewall to sidewall
55 = Aspect Ratio
This is the ration of the height of the tire’s cross-section height to its width. E.g. 55 means that the height is equal to 55% of the tires’s width
R = Construction
The R stands for radial and means the body ply cords - layers of fabric that make up the body of the tire and run radially across it from bead to bead. The letter B indicates that the tire is of bias construction, meaning that the body ply cords run diagonally across the tire from bead to bead, with the ply layers alternating in direction to reinforce one another.
18 = Wheel Diameter
The width of the wheel from one end to the other. The diameter is displayed in inches.This tells you on which rim diameter your tire can be mounted
91 = Load Index
This is an index (not a direct indication of the load) indicating the maximum load that the tire can support when properly inflated.
Y = Speed Symbol