Most modern tires are sized based on the P-Metric system that was introduced in the United States in the late '70s. The method is based on a uniform engineering formula to determine a tire's load capacity, and reflects a tire's section width, sidewall aspect ratio, and diameter. For instance, a P245/50-16 tire has a 245mm section width, a sidewall height that's equivalent to roughly 50 percent of the section width, and a 16-inch diameter. Most muscle cars were built before the P-Metric sizing system was introduced, however, when the Numeric and Alpha Numeric systems were more common. The Numeric system used through much of the '60s measured a tire's section width and diameter, and most featured an 80- or 90-series aspect ratio. For example, a 6.95-14 tire had a section width of nearly seven inches and a 14-inch diameter. The Alpha Numeric system introduced in the late '60s used a letter to describe a tire's load rating, followed by numbers that represented a tire's aspect ratio and diameter. An F60-15 tire used on a second-gen Camaro, for instance, specified a 60-series aspect ratio and a 15-inch diameter. Obviously, the older sizing methods aren't as precise, so trying to convert them to an equivalent tire size on the P-Metric scale can get a little tricky. Corky says the easiest way to do this is to call up Coker's tech support line, or refer to the conversion charts in its catalog. "Tires have evolved tremendously over the years, and so have the standards used to size them," Corky explains. "There have been times in the past when tires were measured by their overall diameter and section width, but the rim size wasn't even specified. Due to all the variations over the years, it's best to consult with a tire expert to see what tire size is right for your car when upgrading to modern P-Metric tires."
Section vs. Tread Width
A tire's section width is the distance between the widest points of the inner and outer sidewall. Tread width, on the other hand, is the actual footprint of a tire from crown to crown. According to Corky, all tires must be mounted on a wheel that's neither too wide nor too narrow to optimize the tire's contact patch and performance. "The Rubber Manufacturers Association and Tire and Rim Association have a recommended range of rim widths a tire should be mounted on," he explains. "The tread must be flat to perform correctly. If too wide, a tire's shoulder will wear, and if too narrow, the center will wear. When operating within the recommended width range, a good rule of thumb to follow is that a difference of 1/2 inch of rim width equates to a 2/10-inch change in section width."
While Coker manufactures its own line of tires, the company started out as a tire distributor. Consequently, it still offers a full line of tires from a diverse range of manufacturers. "We've been in business for over 50 years, and our goal has always been to be a one-stop shop for tires," says Corky. "In addition to our reproduction tires, we carry the entire catalog from BFGoodrich, Michelin, Firestone, Uniroyal, and Phoenix and M&H slicks. Coker also offers steel and wire wheels for antiques and muscle cars, as well as vintage tire related apparel and collectibles."
"When most people build cars, the tires are the last thing they put on their car. Most hot rodders probably know more about what goes into building an engine than what goes into making a tire. However, since tires are the only thing holding your car to the road, you should first figure out the stance, suspension setup, and what tires to run. The hardness or softness of a compound is important to ultimate grip, but that's not the complete story. They may look simple, but tires are highly engineered products. The first step during the design phase is to determine the handling and wear characteristics you want to achieve. To put it succinctly, the tread width, aspect ratio, chaffing materials in bead, compound, sidewall design, cord angles, and the number and type of plies all impact grip. Additionally, chemical engineers have to develop a rubber compound that has good grip and runs cool, since a tire runs best when it isn't overheating. The pressure and temperature used during the curing process is also very important."