In the 1960s and 1970s, the tires installed by the car manufacturer had a letter designation and aspect ratio, according to the Tire Rack website. The letter indicated both the size and load capacity. Tires of the era were bias ply, with tire bodies constructed of cloth belts laid at a angle or bias. Modern tires are radial construction with the belts laid perpendicular to the rolling directing of a tire. Bias and radial tires handle and wear significantly different from each other. Converting the size can be done to a currently used bias ply size or a modern radial tire size.
Locate an online tire size conversion chart. Classic car websites often have a chart showing the different tire sizes and their measurements (see Resources).