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All those numbers and letters that appear on the sidewall of every tire that's manufactured and sold to the public have a very specific meaning. Many of the customers we serve at Texas Tire Sales simply come to the counter and say "I need new tires." Our store manager will go outside, look at the tires they currently have on their vehicle, come back inside and then start rattling off new tire options to the customer. Many times these ramblings are met with a blank stare as if we are speaking some foriegn language. Normally the customer simply responds... "I just want new tires at a good price." Well, that is our main purpose here at Texas Tires Sales... to provide quality tires and auto service at reasonable prices. We are also here to inform customers about the difference between this tire and that tire and help the customer to make the best buying decision for their particular needs. All tires must have certain information molded into them. Some of this information such as the tire brand, series and model, is self-explanatory. The rest is, well, not so much. In reality, it looks more complicated than it actually is.
So, what do all of those numbers and letters mean? Okay, we'll make an effort to break it down as simply as possible, because, when shopping for tires, you need to know what the numbers and letters on your sidewall mean in order to select the correct replacement tire.
For this example, we'll break down a fairly common tire size / description P215/50R17 91V into its individual components. There is, in fact, a lot more information in this nondescript-looking alphanumeric code than many people might think.
We will start with the type and size of the tire, which we determine from the first four characters.
The P at the beginning identifies a tire that has been engineered to U. S. Tire and Rim Association (T&RA) passenger car standards. The lack of a “P” here (as in 215/50R17 for example) indicates a tire that is engineered to European (ETRTO) standards.When a tire size begins with a "P," it indicates that the tire is a "P-metric" size that was designed to be fitted on vehicles that are primarily used as passenger vehicles. This would include cars, minivans, suvs and light duty pickup trucks (usually 1/4- and 1/2-ton load capacity). If there isn't a letter preceding the three-digit tire width number of a tire size, this indicates that the tire is a "Metric" or "Euro-metric" due to the fact that these tire sizes originated in Europe. While Metric tire sizes are mainly used on European cars, they are also often used on vans and SUVs. Euro-metric sizes are dimensionally the same as P-metric sizes, but normally differ slightly in load carrying capabilities.
Other codes you might see instead of "P" used here is “LT”. This tells us that a tire is intended for use on a light truck or SUV, which have heavier load ratings than passenger cars. This includes medium and heavy-duty (typically 3/4- and 1-ton load capacity) pickup trucks, sport utility vehicles and full-size vans. We will cover LOAD RATINGS further down the page. If a tire size ends with "LT," it indicates that the tire is either an earlier "Numeric", "Wide Base" or "Flotation" Light Truck size designed to be used on vehicles that are capable of carrying heavy cargo and/or towing trailers.
If you see a “T”, that indicates that the tire is a temporary or spare tire. If you see "C" it means the tire is a "Commercial" tire meant to be used on vans or delivery trucks that are capable of carrying heavy loads. In addition to being branded with the "C" in their size, these tires are also labeled with their appropriate Service Description and "Load Range" (Load Range B, Load Range C or Load Range D). If a tire size begins with "ST," this means that the tire is a "Special Trailer Service" size that was designed to only be used on boat, car or utility trailers. ST-sized tires should never be used on cars, vans or light trucks or SUV's.
Next is the number 215, This number represents the WIDTH of the tire in millimeters. This is the most important number to describe a tire's contact patch. The 215 indicates this tire is 215 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. The larger the number, the more rubber you have coming in contact with the road. For example, a 215-width, 17- inch tire may look great, but all things being equal, a 245-width, 15-inch tire will out-corner it every time, because the contact patch is wider.
The number 50 - the ASPECT RATIO – This is the height of the sidewall from the rim to the tread, expressed as a percentage of the tread width. Sidewall height is pretty important for a number of reasons:
• When you decide to buy those awesome new 20 inch wheels, a corresponding change needs to happen in the sidewall height of the tire in order for the rolling diameter of the wheel and tire combination to be as close to stock as possible. This will ensure the accuracy of your speedometer and prevent unwanted alignment changes.
• The sidewall height also affects the turn-in feel (the responsiveness you feel at the steering wheel) as wel as the ride quality of your vehicle.
• A lower aspect ratio (shorter sidewall) provides better turning response than a higher aspect ratio, but at the expense of less break-away warning and a more rigid ride. In worst case scenarios, a shorter sidewall may exposed your fancy new wheels to possible bending, breaking or other damage from potholes and other road surface irregularities.
You can calculate the Aspect Ratio (sidewall height) by multiplying the tires width (215) by its aspect ratio (50) with a decimal placed before it (as in “.50”). Therefore, 215 X .50 = 107.5 is the height of the sidewall in millimeters.
If you just HATE the metric system or just have nothing better to do, you can convert this measurement to inches by multiplying the calculated figure by “.03937”. For example, 107.5 X .03937 = 4.23. Multiply the answer by two and add the wheel size as follows (4.23 X 2) + 17 = 25.46. Now, after doing all this math you've determined that this tire is approximately two-feet tall.
OK, moving on... R tells us about the INTERNAL CONSTRUCTION of the tire – how the tires' internal cords are arranged in the tire casing and so on. In this example, “R” stands for “Radial”. Radial tires are by far the most popular type of tire today representing over 98% of all tires sold. Virtually every new tire is a radial, unless you have a special tire for a classic car or you are driving a race car.
Next we have WHEEL DIAMETER. In our example, the wheel diameter number is 17 which indicates that this tire will fit a 17 inch wheel. That was easy.
Up to this point we have covered most of the information that most tire buyers pay attention to or think is important when they are buying tires but wait, there's more... the SERVICE DESCRIPTION. Written on our example as 91V, this tells us a two very important things.
First, 91 indicates the tire's LOAD RATING. This is an assigned numerical value used to compare the load-carrying capabilities of any given tire. In general, higher numbers indicate greater load carrying capacities. 1356 pounds In this case, 91 means the tire is rated for 1,356 pounds (see Table below). Multiply this by the number of tires on the car, and you get a maximum safe loaded vehicle weight of 5,424 pounds (including people, animals, a full tank of gas, and all the bobble heads on the dash board).
Load Rating Designations
Second, and just as important, is the SPEED RATING for the tire. This is also an assigned value system. In this case, V indicates this tire is approved for speeds up to 149 mph.(see table below). Will this tire come apart if you drive at 155 MPH? Probably not . At least not as soon as you hit 150 MPH but, it might if driven at 155 MPH for a long period of time. Better safe than sorry.
In the past it was acceptable to publish a speed rating symbol “within” the tire size data itself (for example, P215/50VR17). In 1991, the speed rating letter was moved to its current location on the sidewall. You still might see it within the tire size data on some tires.
It's almost impossible to find a public road in the U.S. where you can LEGALLY drive faster than any tire's rated speed, but speed ratings nonetheless are an important safety consideration when shopping for tires. Especially if you are the type of person that has no concept of speed, doesn't pay attention to signs or you plan on driving your car in Germany.
Speed Rating Designations
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 vehicles doorjamb.
TRACTION RATING A traction rating can also be found on the sidewall of all your 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 that you need to avoid tailgating when driving on wet surfaces.
TEMPERATURE RATING It doesn't take a rocket scientist to understand that friction causes tires to heat up as you drive down the road. The temperature rating represents the ability of the tire to withstand heat at high speeds. The ratings, from best to worst, are: A, B and C.
TREAD WEAR RATING You might find the word "TREADWEAR" on the sidewall followed by a number like 120 or 180. This number is a rating of how durable the tire tread is as tested against the industry standard. The reference number is 100, therefore, a tire with a tread wear rating of 200 has an 80 percent longer anticipated tread life, while a rating of 80 represents a predicted tread life only 80 percent as long as the industry standard.
Well, there you have it. If you are ever bored someday and are looking for something interesting to read, don't bother going to Amazon.com. Just go outside and read your tires.
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