“Mags & Meats”… Well, that’s what we called them in the Seventies By John Novak When I started all of this car stuff, tire sizes were pretty simple, there just weren’t any choices. I remember the day I took off the H60’s and replaced them with the fattest tires in SoCal…N50’s. Everything was bias ply and the only radial tires were on your dad’s Buick Electra, but none of the gear heads used them. Now there is a staggering array of tire and wheel size and profile choices. Let’s decode a typical car tire; something like this “P165/60R 15 82 XXX H.” A ton of information in that little code, but it doesn’t do you any good if you don’t know what it all means. Think of it as the VIN Code for a tire. The first letter tells us what type of vehicle the tire is intended for. LT is for light truck, P indicates passenger car, and T is for temporary, can you say “Donut” spare tire? The next set of numbers, “165”, in this sequence indicates the section width of the tire (in millimeters). Taller or wider tires would have higher numbers. The numbers after the slash mark (“/”) indicate the aspect ratio of the tire. The aspect ratio is actually the tire’s section height Vs section width as a percentage. In our example the height is 60 percent of its width. Most performance tires would have a lower number in this space. The following letter in our example stands for the tire’s type. In this case, “R” stands for radial. The “15” immediately following the “R” is the diameter of the wheel (in inches). The next two numbers in our example are “82” and indicate the tire’s load index. By referencing an on line Load-Carrying Capacity chart, I found the rating to be 1,047 pounds; as a result a set of four tires would safely support a vehicle weight of 4,188 pounds. The next letters or numbers (XXX in my example) indicate the manufacturers code for the tread design etc. The final letter stands for the speed rating. The speed ratings are: S = 112 mph, H = 130 mph, V = 150 mph, Z = +150 mph. As speeds increase the tire generates more heat than it can get rid of and centrifugal force can actually pull the tread from the belts. Not a good time for that to happen! Remember, increasing the tires diameter also increases the torque loads (leverage) placed on components like axles, u-joints and driveshafts. Often times moving to larger tires makes it necessary to upgrade these areas. Increasing the width of the tread also places increased loads on all components when limited slip or locker differentials are used. Also bigger tires are usually heavier than the stock treads. The increased rotating mass also increases the requirements in braking power. Sometimes this is accomplished by simply changing brake pad materials. It may require disc brake conversions, or even switching to larger performance disk and calipers. Increased tire/wheel weight also increases unsprung weight in the suspension geometry and may require upgrades to shocks and springs. This does not matter much on the street, but can make the ride at highway speeds very harsh and uncomfortable. Extremely wide tire and wheel combinations are much more difficult to balance, especially those with large offsets. This is because of the extra weight of the tire. The increased rotating mass of the larger tire has a flywheel effect, and the tire and manufacturers tolerances might make it impossible to place enough balance weights to bring it true. This is very common when the wheels are of a very light alloy. Sometimes it is necessary to juggle tire & wheel combos to find a good marriage. Now let’s decode the Wheel Size, in our example 15x7 5-115 40S breaks down like this: 15 indicates the size of the wheel measured across its center. The 7 indicates the width of the wheel when measured across its barrel section from the front to the rear rim. The 5-115 indicates the bolt pattern. 5 lugs placed on a circle circumference of 115 Millimeters. (The circle would cross the center line of each lug hole) 40 indicates the offset of the wheel’s centerline Vs hub mounting surface + or -. Zero Offset = Mounting surface is centered on the wheel. Positive Offset = Mounting surface is moved to the outside of the wheel. Most OEM designs use this style to give maximum brake clearance, and to aid in fender well clearance. Negative Offset = Mounting surface is moved to the brake side of the wheel. This is referred to as “Deep Dish”. The final letter or number would indicate the manufactures finish. In this case Silver. This is only the basics, and opening a can of worms like “Plus Sizing” the mixing & matching of tire and wheel combinations is almost endless today! Now that you have the new custom tire and wheel combination mounted on your ride, how do you know the correct tire pressure? A good way to start is to sprinkle some white powder on the shop floor, baking soda works great. With the tires cold, drive across the powder and check the treads, those that contact the pavement will have powder on them. Adjust the air pressure until you get a good contact with all the treads across the tire. Use this as a base line, and adjust as you start to see the wear patterns. Now you are ready to side step the clutch and fry’em! Remember... The print version of CruZin' Magazine contains the content above, plus MANY features not available online. Pick it up at your local newsstand, or subscribe today and never miss another issue! Home | Club Roster | Calendar | Classifieds | Connections | Archives Subscribe | Advertise | Interact Copyright, 2005. Island Enterprises. All Rights Reserved.