Among the many reasons our friends at Hagerty are a wonderful partner to the SCCA is that they love #FunWithCars just as much as we do – and they are a wealth of knowledge on a variety of car-related subjects. The following article originally appeared on Hagerty.com. For this and all of Hagerty’s car-related content, visit their media site here.
Until about 1920, cars wore skinny high-pressure tires in sizes such as 30×3, where “30” was the tire’s rolling diameter and “3” was the cross-sectional width, both in inches. To calculate the diameter of wheel they’d fit on, you needed to know that the tire’s width and height were the same; there are 3 inches at both the top and bottom, so subtracting 6 from 30, the tire would need a 24-inch wheel.
From the 1920s through the 1960s, a numeric description containing tire width and wheel diameter was used. A size such as 6.00-16 referred to a tire 6 inches wide made for a 16-inch wheel. Tires might have been available in different profiles, but it wasn’t part of the description.
The designations changed again in the late 1960s when alphanumeric descriptions came into play. A size like G70-15 meant the tire had a g-load rating (1620 pounds), a 70 percent aspect ratio (the tire’s height divided by its width, expressed as a percentage—the smaller the aspect ratio, the lower the tire’s profile), and was meant for a 15-inch wheel. Width was no longer part of the description.
Finally, in the late 1970s, P-metric tire designations were introduced. They included the three important parameters of width, aspect ratio, and wheel diameter, but, confusingly, width and diameter are specified in different units. If you see a tire size like 235/45R17, the 235 is the section width in millimeters, the 45 is the aspect ratio, and the 17 is the wheel diameter in inches.