Going in the 1930's the most common tire size was 6.00-16. Again the 6.00 refers to the section width of the tire, resulting in a tread width of about 4-1/2 inches. After World War II, the automotive world began changing, with futuristic designs, creature comforts and more powerful engines.
As time went on, American cars gradually transitioned into smaller wheel diameters, while increasing tire and wheel width. This provided better performance, as the wider contact patch and shorter sidewall helped with traction and stability.
As a general rule of thumb, tires from the 1930's through the 1950's feature a 90 percent aspect ratio, meaning that the sidewall section height is 90 percent of the tire's section width. Any tire size that ends with a zero, such as 6.70-15 or 7.10-15 featured this approximate high percent aspect ratio. By the mid 1960's, tire sizes transformed again. Any bias ply tire ending in the number five, such as 6.95-14, 7.75-15, etc. featured an 80 to 84 percent aspect ratio.