Regarding tires, there are at least a couple of things that we know for sure. First, they come in all sizes and types, depending on vehicle type, season, and traction requirements, to name a few. Second, tires don’t last forever, from as little as 20,000 miles to 80,000 miles, and will, sooner or later, require replacement.
When it comes time for replacement, there are typically two ways to go about it. One way is to simply ask your trusted technician for replacement tires, all-season tires for a 2005 Toyota RAV4, for example, and he’ll look up a list of tires that are designed to fit the vehicle. The only thing you, as a customer, might have to do is choose between brands and models. On the other hand, you could go out shopping for tires on your own, at which point it becomes a little more complicated.
Of course, there are the usual tire brand and model considerations, but there are many different sizes and applications, signified by a string of numbers, letters, and symbols on the side of the tire. For example, on the sidewall of a performance tire, perhaps for the BMW 3-Series, you might see something like this: Kumho Ecsta KU31 SPT P255/40R17 94W RFT. An off-roading tire, perhaps for the Toyota HiLux, might look something like this: Bridgestone D673 DUELER LT33/12.50R15 108Q OWL. A lot of other information may also be imprinted in the tire sidewall, as well as symbols, such as a snowflake, sun, or water drop. What does it all mean?
The Basics of Tire Size Designations
Aside from the brand and model designation, the numbers may be the most relevant part of shopping for a tire. The first letter, if present signifies whether the tire is designated for a passenger car or light truck. In our two examples, P255/40R17 is for a passenger car, and LT33/12.50R15 is for a light truck. Looking at the two tires, it might seem obvious, but the information is there for comparison. The next part of the numbering refers to the size of the tire. To note, the “/” serves as a separator and, contrary to popular misconception, “R” does not refer to Rim diameter, but to the fact that the tire uses Radial construction, as opposed to older bias construction types.
In the case of the P255/40R17 passenger tire, the first number refers to the width of the tread in millimeters. The second number refers to the height of the tire, a percentage of the width of the tire. The final number refers to rim diameter, in inches. This particular passenger tire, therefore, has a tread width of 255 mm, tire height of 102 mm, and fits on a 17-inch rim.
In the case of the LT33/12.50R15 light-truck tire, the first number refers to the overall height of the tire, in inches. The second number refers to the tread width, in inches, and the final number refers to rim diameter, in inches. This particular light-truck tire, therefore, has a tread width of 12½ inches, a height of 33 inches, and fits on a 15-inch rim.
Load and Speed Designations
Depending on application, these numbers can vary widely. After the tire sizing data, the next number-and-letter combination refers to load and speed capability of the tire at 42 psi. Keep in mind, however, that each particular vehicle has its own recommended tire pressure specifications, and that each vehicle and driver has their own limitations when it comes to speed and handling.
In our passenger tire example, 94W coincides with two conversion tables. The “94” load rating of the tire signifies that the tire is designed to carry a maximum load of 670 kg (1,480 lb) per tire. The “W,” noting that this is a performance tire, signifies that the tire is designed to operate safely at speeds up to 270 km/h (168 mph), when at its maximum load. In the case of the light truck tire’s “108Q” rating, this signifies that the tire is designed to operate safely at speeds up to 160 km/h (100 mph) at a maximum load of 1,000 kg (2,200 lb) per tire.
In the case of our two example tires, there are a few other designations. On the passenger tire, for example SPT is short for Sport and RFT is an acronym for Run Flat Tire. The light truck tire’s M+T is for Mud + Terrain and OWL is for Outlined White Lettering. Some tires may feature a snowflake icon, designating tires designed to run in cold weather, while a sun icon designates tires designed to run in hot weather. Directional-tread tires, such as special rain tires or performance tires, will have arrows indicating the proper direction of travel for the tire to perform its best. Similarly, asymmetrical-tread tires usually have SFI and SFO (Side Facing In or Out), or Inner and Outer, to show in which position the tire should be mounted on the vehicle for best performance.
Here are a few other designations found on many tires:
BSW or WSW indicate Black or White Sidewall
OWL, RWL, or ORWL indicate Outlined or Raised (or both) White Lettering
TL or TT for Tubeless or Tube-type tires
TWI, or an outward-facing arrow, indicate location of Tread Wear Indicators
LL, SL, or XL refer to Light, Standard, or Extra Load tire construction
Various manufacturers may have certain model pre-approved as original equipment tires, such as Mercedes-Benz (MO), BMW (a star), or Jaguar (J), to name a few.
Red Circles indicate where the tire is most elongated, and Yellow Dots indicate the heaviest spot on the tire. Both of these painted marks come in handy for the technician mounting and balancing the tire.
Resizing Rims and Tires
Sometimes, when buying new tires for your particular vehicle, you may want to go with a different size, such as wider tires for better summer traction. Simply going for a bigger first number could throw off your speedometer reading, because the speedometer is calibrated for a certain diameter tire. Another common practice is getting bigger rims to reduce tire height, so a tire with a smaller ratio, the second number, needs to be chosen to maintain tire diameter, speedometer readings, and component clearance.
Take, for example, replacing the original tires and rims on a 2006 BMW 325xi Sport Wagon. The original tires are Continental ContiSportContact 2 SSR in size 225/45R17 91V RFT. To improve handling, cornering, and possibly braking, upsizing to 20-inch wheels might be a good choice. Putting 225/45 tires on a 20-inch rim, however, would result in too tall of a tire, throwing off the speedometer readings, and may not even fit on the vehicle without modification. To arrive at the correct tire height, perhaps a good choice in tire would be the Continental ExtremeContact DWS 245/30R20 90Y, 20 mm (0.8 in) wider, for good traction, and just 20 mm (0.8 in) taller. Most professional tire sellers can help you determine the right tires for your particular ride.