Magazine for the professional tire industry
Issue link: https://mtd.epubxp.com/i/664171
Quik-Link: 800-687-1557 ext. 15110 1. Follow the 3% Rule. Since the goal of most custom tire and wheel packages is to switch from the original equip- ment size to a larger tire, the best place to start is the 3% rule. Years ago, several tire manufacturers recommended that plus-size ftments remain within 3% of the diameter of the OE tire specified on the placard. Since then, very few, if any, have continued to publish that guideline, but it still exists in the Tire Industry Associa- tion's (TIA) Automotive Tire Service (ATS) Program and will remain there because it has been used in a number of legal cases. There have also been a number of tests in the past that showed exceeding the 3% guideline can have a nega- tive effect on vehicle stability, electronic stability control, and other ride or braking systems. In order to illustrate how it works, let's look at a real world example. The 2014 Cadillac Escalade has an OE tire ftment of P265/65R18 112S. According to the Tire and Rim Associa- tion (TRA) Annual Yearbook, the P265/65R18 has an outside diameter of 31.54 inches. Since the 3% rule is a plus/minus, the retailer just needs to perform simple math to get the range of acceptable diameters: 31.54 x .97 = 30.59 and 31.54 x 1.03 = 32.49. This means that sizes other than the OE fitment should have an outside diam- eter between 30.59 and 32.49 inches. The other factor that needs to be considered is the load index. The OE tire has a load index of 112, so the replacement tire must have a load index that is equal to or higher than 112. This isn't a "should" or a recommenda- tion. Retailers can never install a replacement tire with less load carrying capacity than the OE specifcation. A c c o r d i n g t o t h e R M A , "Replacement tires should be the same as the OE size desig- nation, or approved options, as recommended by the vehicle or tire manufacturer." Since the 3% rule comes from the tire manu- facturers and the guideline itself is a "should," plus-sizing is allowed as long as certain restrictions are followed. A f e w y e a r s a g o , I w a s involved in a legal case where the custom tire was .12 inches over the 3% threshold for the original equipment tire. That was more than enough for the plaintiff to suggest it was not compliant with industry guide- lines. Never mind the fact that a tenth of an inch in diameter amounts to a few millimeters difference in ride height. I was representing the whole- saler who sold the tires to the retailer and maintained that the installer was ultimately respon- sible for ftment decisions. It 18 Feature