The information found on a tire sidewall contains important information about the tire size, load capacity, weather ratings, and even government ratings for treadwear, traction and temperature. For illustration purposes, we will look at a 245/45ZR-20 tire (typical size for a Challenger R/T).
- Section Width
The 245 indicates that the tire is 245 millimeters across from the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified width wheel. Generally speaking, the larger this number is, the wider the tire will be.
- Aspect Ratio
Aspect Ratio is the ratio of the sidewall height to the section width. The sidewall height of the example tire above is 45% of its section width. This number can be very indicative of a tire's purpose. Lower numbers, like 40 or less, mean a shorter sidewall to accommodate larger tires for increased handling and traction (see Rim or Wheel Diameter below)..
- Rim or Wheel Diameter
Wheel Diameter specifies the size, in inches, of the wheel that a tire fits. The example tire will only fit a 20-inch wheel. Pay particular attention to this number if you plan on upgrading your wheel size. If your wheel diameter changes, you'll have to purchase a new set of tires that matches the new diameter. Similarly, if you want to install wider tires on your existing wheels, make sure that the tires are the same diameter to keep your speedometer accurate. You can accomplish this by buying tires with compatible aspect ratios. For example, a 245/45-20 tire has the same diameter as a 275/40-20 tire.
- Internal Construction
The "R" refers to radial construction, which has been the industry standard in passenger-car tires for more than 20 years. Prior to radial tires, most cars came with bias-ply tires, which had a crude construction that made for poor handling. Bias-ply tires (which use a "B" for their description) are still used for certain truck applications.
- Load Index
A tire's load index is a measurement of how much weight each tire is designed to support. The larger the number, the higher the load capacity. This is one of the most important numbers on your tire. Remember that this is per tire, which means you have to multiply by four to get the total capacity for a complete set of tires (see chart, below). If the vehicle has its original tires, you can just refer to the doorjamb, which lists the maximum cargo capacity with passengers. Why is this important? Large-diameter wheels with lower-profile tires tend to have less load-carrying capacity because they contain less air. And it is the volume of air inside the tire, not the rubber itself or the wheel material, that shoulders the load. In the example of the 245/45-20 tire, the load index per tire is 1,929 lbs (or 7,716 for all four tires- this is more than enough capacity to support a 4,200 Challenger with passengers).
- Speed Rating
The speed rating is a measurement of the speed at which the tire is designed to run for extended periods. A "Y" speed rating signifies that this tire can be run safely at speeds of up to 186 mph for extended periods.
Here is a complete list of the various tire speed ratings, and their associated letters:
- S 112 mph
- T 118 mph
- U 124 mph
- H 130 mph
- V 149 mph
- *Z Over 149 mph
- *W 168 mph
- *Y 186 mph
- *(Y) Over 186 mph
- *The "Z" rating used to be the highest rating for tires having a maximum speed capability greater than 149 mph, but as tire technology improved, it is was ultimately split into the "W" and "Y" rating. A "ZR" may sometimes appear in the size designation, as a sort of nod to the prior rating, but it will also be used in conjunction with a W or a Y.
- DOT Code
The DOT code is used by the Department of Transportation (DOT) to track tire production for recall purposes. If a tire proves to be defective, this number helps keep track of where these tires ended up so that buyers can be notified of the problem. At the end of the DOT code you'll find a four-digit number. This is the manufacturing date of the tire. The first two digits stand for the week; the other two are the year. For example, if your tire had "1617" listed, it was manufactured on the 16th week of 2017. (Note- Tire experts recommend that tires that are six or more years old be replaced, regardless of tread depth). Sometimes the DOT number will be located on the inside of the tire. In this case, you can either jack up the car to inspect it, or check with your local mechanic or tire shop. You should also make a habit of checking the manufacturing date on your spare tire as well.
- Maximum Air Pressure
This number refers to the maximum amount of air you can put in a tire before you harm it. It is not the recommended tire pressure; that number can be found in your owner's manual and on the doorjamb.
- Traction Rating
A traction rating can also be found on the sidewall of all modern tires. It can be represented as AA, A, B or C. This is a rating of a tire's traction when tested for straight-line braking on a wet surface. For this rating, AA signifies the best traction performance and C indicates the worst.
- Temperature Rating
The temperature rating refers to the ability of the tire to withstand heat under high speeds. The ratings, from best to worst, are: A, B and C.
- Treadwear Rating
Finally, you might find the word "TREADWEAR" on the sidewall followed by a number like 120 or 180. This is a rating of the tread's durability, as tested against an industry standard. The reference number is 100, so a tire with a treadwear rating of 200 has an 80% longer predicted tread life, while a rating of 80 means a shorter predicted tread life of only 80% percent the industry standard.
The standard tire on a 2018 Challenger R/T Scat Pack is a 245/45-20 Goodyear RS-A all-season tire. This is a hybrid that visually looks like a cross between a three-season and a winter tire. It blends dry and wet road performance with wintertime slush and snow traction, while providing ride quality, smooth handling and durability. Several excellent replacement tires are Bridgestone Potenza RE 980AS, Continental Extreme Contact DWS 06, Michelin Pilot Sport A/S and Cooper Zeon RS3-G1.
Here are two photos of a 275/40-20 Cooper Zeon RS3-G1 all-season tire installed on a 2009 Challenger SRT, with 9-inch Alcoa forged aluminum wheels:
Two three-season options on a Challenger are a 245/45-20 Goodyear Eagle F1 (no cost) and a 275/40-20 Pirelli P Zero ($695 option). These tires provide maximum traction and performance. However, unlike the all-season tire, they should not be driven when the temperature falls below 45F degrees. Summer performance tires feature tread compounds engineered to provide traction in warm to hot temperatures. They were never intended to experience near and below-freezing temperatures, or the winter driving conditions. As temperatures get colder, they lose a noticeable percentage of traction, as their tread compound rubber changes from pliable elastic to inflexible plastic (e.g., like a frozen hockey puck). If temperatures drop to near or below-freezing, driving a vehicle equipped with summer performance tires actually becomes dangerous. It also risks the possibility of tread compound cracking or a chipping away of the edges of the tread blocks.
Several excellent replacement summer tires are Bridgestone Potenza RE-71R, Dunlop Direzza Zlll, Falken Azenis RT615kt, Hankook Ventus R-S4 and BF Goodrich g-Force Rival 1.5.