In the case of a 1930 or 1931 Ford Model A, the original tire size was 4.75-19. It leads off with the tire's section width, in inches and then features its rim diameter (ex. When this new style tire and wheel hit the market, it had a new sizing structure that we now refer to as numerical sizing. Early Balloon tires, like the ones that were optional on later Ford Model T's, changed the automotive industry. This started with the tires that were marketed as Balloon tires, due to their lower air pressure and soft ride. Numeric sizing is most common on bias ply tires manufactured from the late 1920’s through the late 1960’s. They would eventually be phased out by Balloon tires and drop center rims, which used less air pressure. High pressure tires are common on vehicles built before 1927. In this example, our quick math tells us that it fits a 25-inch rim. To determine the rim size on high pressure tires, we double the section width number (3 + 3 = 6) and then subtract it from the diameter (30 - 6 = 24 inches).Īnother example is 34 x 4 1/2 (these are always written out in fraction form, not decimals). The "30" in the sizing refers to the tire's overall diameter and the "3" is an approximation of the tire's section width-simple enough, right? The only thing left out of the size is the rim size. Whether it's a clincher rim, lock ring style or other type of Brass Era rim, these high pressure tires feature an early sizing style.Ī common size for a high pressure tire is 30 x 3. We now refer to these tires, such as the Goodrich Silvertownd Cord (pictured), Firestone Non Skid and others, as "High Pressure" tires. When pneumatic tires hit the mainstream in the 1900's, tires were inflated to 65 psi and higher, in order to hold the tire to the rim properly.
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