Tire Size 101

In this article, I’m going to cover a topic that seems straightforward, but is widely misunderstood throughout the collector car world. That topic is the evolution of tire sizing. Standards have changed over the past 100 years of tire manufacturing and the result is multiple versions of sizing terminology. I’m going to cover the basics here, but you can always log onto our website for more in-depth information. For an expanded version of this tire size tutorial, go to www.cokertire.com/tire-size.

There are three common types of tire sizing from the 1920’s through the modern era—numeric, alphanumeric and P-metric. Uncommon forms of tire sizing are used for high pressure tires (Brass Era), racing tires, vintage metric tires and certain types of off-road truck tires. Let’s cover the three most common tire sizing styles.

Numeric sizing is most common on bias ply tires manufactured from the late 1920’s through the late 1960’s. This started with the tires that were marketed as Balloon tires, due to their lower air pressure and soft ride, compared to the earlier high pressure tires of the 1900’s and teens. Numeric sizing leads off with the tire's approximate section width, listed in inches and then features the rim diameter (ex. 6.70-15 or 8.00-14).

Alphanumeric sizing became common in 1967, and was the industry standard only a few years later. This type of sizing takes the guess work out of the aspect ratio, but it creates some guess work on the tire’s overall width. The size starts with a letter, which is the tire's load range. The higher the letter, the larger the tire. A78-13 is an example of a very small tire for an economy car, and L78-15 is an example of a large tire for a Cadillac. The two-digit number after the load range rating is the aspect ratio. The introduction of alphanumeric tires also saw the introduction of smaller aspect ratios, and shorter sidewalls. 78 series was common, but 70 series and 60 series came shortly after. 

The muscle car era saw creative usage of alphanumeric tires, including the Firestone Wide Oval. It was available in redline originally and then later in raised white letter. Popular sizes for the Firestone Wide Oval was F70-14 and G70-15. Aftermarket tire companies, such as Pro Trac tires offered custom alphanumeric sizes, such as N50-15, an enormous tire that was typically used on the back of modified muscle cars. Radial tires were introduced to American markets in the late 1960's and became original equipment offerings in the mid 1970's. The alphanumeric sizing remained for a short while, but an "R" was added after the load rating (ex. LR78-15). 

The most current form of tire sizing is P-Metric. This sizing nomenclature can be confusing because the section width is listed in millimeters, the aspect ratio is listed as a percentage and the wheel diameter is most often listed in inches. For example, a P205/75R15 tire features a section width of 205 millimeters, and a sidewall that is 75 percent of that width. The “R” tells us it’s a radial and the 15 indicates a 15-inch wheel diameter. P-Metric radial tires are a common replacement for original bias ply tires. In the case of 1950's classics, we most often suggest a 205/75R15 for cars that originally came with a 6.70-15 bias ply tire. We also use 205/75R15 radials as a cross reference to original 7.75-15 Corvette tires. We’re proud to offer thousands of tires for collector cars of all ages. We have new product lines being added regularly, expanding our list of applications and potential customers. We’ve recently introduced new product lines in the vintage sports car market, which I plan to cover in a future column.