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  • Tire Tech 102: Numbers and Ratings

    In the previous course -- Tire Tech 101: Dimensions -- we explained what some of the most common information printed on a tire's sidewall meant.  Now that you've aced that class and are ready to graduate to the big leagues, we're going to look at some of the smaller print on your tires, and also chat a little about tire designations you might see on various websites or in the stores.

    But first, a minor digression:

    Uniform Tire Quality Grading System, or UTQGS

    The United States government established the UTQGS (Uniform Tire Quality Grading System) to assist consumers in understanding -- at least at a basic level -- the quality of the tires they can buy.  It looks at three areas:

    • Treadwear
    • Traction
    • Temperature

    These three areas are graded during safety and inspection tests, and are printed both on the tire's sidewall and on the paper labels affixed to each tire's tread when they are to be sold to consumers, so that you can easily find the information.

    So, digression over, here's where Tire Tech 102 truly begins:

    Tire Ratings

    Tire Ratings

    Treadwear

    This is a comparative rating -- usually within brands -- of the useful life of a tire's treads under controlled conditions.  Because one manufacturer may grade something as a 400 (which means it will last twice as long as a tire graded 200) and another manufacturer might grade essentially the same tire as 300 (lasts twice as long as a 150 grade wheel), it's not very useful when comparing tires made by different companies.  And you have to take into account that you probably won't be driving in scientifically controlled conditions, so who knows what kind of elements might reduce tread performance.

    Tread Depth

    Tread Depth

    Traction

    Graded AA, A, B, or C (from highest to lowest, respectively), this is a measure of the tire's stopping ability on wet pavement.  This doesn't take into account turning performance or any vehicle-based stopping performance, and is yet another test done under scientifically controlled situations, so it's really just a comparison rating.

    Temperature Resistance

    Resistance to heat and ability to dissipate heat are important  because tires are constantly under friction (which, as any physicist knows, creates heat), and surrounded by even more friction (brakes on wheels, for instance).  The grades from highest to lowest are A, B, and C, with C being the minimum required performance under federal safety standards.

    Other Designations

    There are several other designations, some of which can be found on the tire, others which might not be.  These include:

    • Service Designation: A designation of "M&S" somewhere on the tire means it is rated by the manufacturer as suitable for use in mud and snow, with guidelines set by the Rubber Manufacturers Association (RMA) in the U.S.
    • All-Season Designation: This is basically saying that the tires meet the "M&S" service designation (above) without the drawbacks of noise or rolling resistance that comes with some winter tires.  Passenger and light trucks might also meet criteria that provide superior snow performance, usually designated by a mountain/snowflake symbol on the tire.
    • D.O.T. or DOT Code: This is a numeric code (sometimes alphanumeric) that indicates the manufacturer, plant where the tire was produced, tire line, tire size, and the week/year of manufacture.
    • Maximum Pressure / Load: All tires are marked on the sidewalls with the maximum load capacity in pounds, as well as the maximum inflation pressure (just look for "P.S.I.").  Truck tires have dual and single application recommended pressure for maximum loads.

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  • Tire Tech 101: Dimensions

    We don't always show off the "and Tire" portion of our name, L.A. Wheel and Tire, but it's a pretty important part of what we deal with every day.  In a very real sense, all the wheels in the world couldn't do very much without tires (or maybe tracked treads, but that's neither here nor there).

    Tire manufacturers are pretty smart people -- engineers, designers, and the like -- and they figured out that they could save a lot of people a lot of hassle by putting some information directly on the sidewall of every tire.

    In Tire Tech 101 -- the first of our series on tires -- we'll give a tour of some of the most important bits of information that you can find on your tires, and what it all means.

    Looking at these pictures, you see the following information:

    Tire Dimensions

    Tire Dimensions

    P235/60R15 98V

    P-Metric / Non-P-Metric Designation

    If there's a "P" appended to the beginning of the information, this simply designates its a passenger car tire.  If there's no "P" at the beginning, the tire is simply engineered to different standards.  The standards are set by two organizations: the T&RA (The Tire and Rim Association) and the ETRTO (European Tyre and Rim Technical Organization).

    Section Width

    The next number ("235" in this case) measures the tire section width, in millimeters: the distance from sidewall to sidewall.  Each tire is measured to specific rim width, and because rim width is often measured in inches here in the U.S., it's helpful to convert millimeters to inches.  Divide the millimeters by 25.4 to get the measurement in inches.

    Aspect Ratio

    After the slash (/) is the aspect ratio (in this case, "60"), which compares the tire's section height -- the distance from bead to the tread -- to maximum section width.  So, in this case, the 60 means that the tire's section height is 60% of the tire's section width.

    Construction

    A letter designation follows, indicating the type of ply construction in the tire's casing.  "R" in this case stands for radial, while other designations are "D" for diagonal and "B" for belted.  Don't mix construction types on a car; you'll regret it pretty fast!

    Rim Diameter

    This diameter is the rim diameter, usually in inches.  Always match the tire's rim diameter to the wheel's rim diameter for safety and proper fit.  In this case, the number is 15, so that should tell you that the wheels this will fit on are 15" wheels.

    Service Description

    This is actually an alphanumeric combination (in this example, "98V") that gives you both the load index and the speed rating.  The Load Index tells you the load carrying capacity of the tire, and is repeated on all U.S. tires with the load limit in pounds elsewhere on the tire.  The Speed Rating is the maximum speed that the tire is rated at the load specified by the load index.  So, the example tires are rated at a "V" which means up to 149 mph (240 km).  Speed ratings work like this:

    Speed Rating / Test Speed

    • Q / up to 100 mph (160 km)
    • R / up to 106 mph (170 km)
    • S / up to 112 mph (180 km)
    • T / up to 118 mph (190 km)
    • U / up to 124 mph (200 km)
    • H / up to 130 mph (210 km)
    • V / up to 149 mph (240 km)
    • W / up to 168 mph (270 km)
    • Y / up to 186 mph (300 km)

    It's important to note that not all U.S. tires are speed rated, but most areALWAYS replace tires with equal or higher rated tires, otherwise you could be looking at some dangerous changes to performance.  Furthermore, regardless of what a tire says it's rated for, you should never break the speed limit, and you should never try to meet or exceed the listed speeds by rating.  Performance and handling can all take a hit under various circumstances, and frankly, crashing your car is no good for anybody.  Except maybe your insurance provider.

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