As your car’s only four contact patches on the road, tires play an important role in keeping you safe and secure no matter the road or weather condition. Therefore, it’s important to know a bit more about your tire, especially about the various markings found on the sidewall. Though it’s so easy to dismiss these numbers and symbols, they can tell you a lot about your tire.
#1: Tire Size Description
This is often the first think looked at by would-be car buyers because it contributes hugely to the form or porma factor. This set of numbers, for example, 235/50R18, gives you the tire’s width, sidewall aspect ratio, and diameter.
In this case, “235” is the tire’s width in millimeters measured from the widest point of the outer sidewall to the widest point of the inner sidewall when mounted and measured on the specified width wheel. This measurement is also referred to as the tire’s section width. Take note that because tires do deform during mounting, so the overall width may vary from one tire make to another even if they carry the same section width.
The “50” meanwhile signifies the sidewall height measured from the rim to the tread. In this case, it’s 50 percent of the tire’s section width. The lower the number, the lower the sidewall.
Next, we have the “R” which refers to the tire’s type of construction. In this case, “R” refers to a radial construction where the tire’s plies radiate out from the center of the wheel. Though fitted to a vast majority of vehicles, some still use “D” or diagonal construction where the plies crisscross. This is known as “bias ply” construction and is commonly found in trucks and commercial vehicle applications.
Finally, we have the diameter. “18” here indicates that it’s meant to be mounted on 18-inch wheels.
#2: Speed Rating
Located usually in-between the section height and diameter (235/50ZR18) or appended after the long tire size string (235/50R18 Y) is a curious letter that dictates the tire’s speed rating. With some highways, particularly in Germany, derestricting speed limits, it’s important to know the maximum speed your car can go. Speed ratings are determined using laboratory tests and only apply to tires that haven’t been damaged, altered, underinflated, or overloaded. A tire which has been repaired can no longer retain the original manufacturer’s speed rating. Here’s a quick reference on your tire’s maximum speed:
• S: 180 km/h
• T: 190 km/h
• U: 200 km/h
• H: 210 km/h
• V: 240 km/h
Curiously, when “Z” rated tires were first introduced, they were thought of reflecting the highest tire speed that would ever be required. Ultimately, as cars became faster, the industry added two new standards:
• Z: in excess of 240 km/h
• W: 270 km/h
• Y: 300 km/h
Now, even the “Y” speed rating has been segmented further indicated by parenthesis. When the “Y” is enclosed in them, say “(97Y)” that means they’ve been tested in excess of 300 km/h.
#3: Load Index
Commonly overlooked when upgrading tires, the load index is a numeral value that indicates a tire’s load carrying capability. Usually found in a range from 70 upward, the higher the number, the greater the tire’s carrying capacity. If upgrading tires (whether they be a direct replacement or going plus sizing), it’s important to specify tires with the same or higher load index than the Original Equipment (OE).
#4: Max Inflation
Pretty much self-explanatory, the “max inflation” figure indicates the maximum inflation pressure that the tire can safely take. It’s typically measured in PSI or Pounds per Square Inch, though some tires may have it in Kilopascals (kPa) or bars of pressure (bars).
Take note though that this does not indicate the recommended tire pressure you should be running on your vehicle. Consult your owner’s manual or the vehicle’s tire inflation placard (usually found in the driver’s door jam or in the inside fuel tank cap) for the recommended tire pressures.
Interestingly, owners running correctly plus sized tires need not worry about computing for the right tire pressure. If done right, there will the same volume of air present in the tire despite the increase in diameter and width, and decrease in size wall height. But for an extra layer of security, adding 2 PSI won’t do any harm.
#5: UTQG Ratings
UTQG or Uniform Tire Quality Grade Standards are provided to provide buyers with useful information that can help them purchase tires based on their requirements: treadwear, traction, and temperature capabilities. The grades are assigned by the tire manufacturers based on their own internal tests or those conducted by an independent testing company that they have hired.
Unfortunately, the rating that is of the most interest is the one that’s also the most inconsistent. While the “Treadwear” grade was originally intended to be assigned purely scientifically, it has also become a marketing tool used by manufacturers to help position and promote their tires.
UTQG Treadwear Grades are based on actual road use in which the test tire is run in a vehicle convoy along with standardized Course Monitoring Tires. The vehicle runs a prescribed test loop with its alignment set, air pressure checked, and tire rotated 1,200 kilometers. The tire is then measured against the Monitoring Tire at the end of the test. The Monitoring Tire is then assigned a grade and the particular tire is graded indicating its relative treadwear. A grade of 100 would indicate that the tire tread would last as long as the test tire, 200 would indicate the tread would last twice as long, 300 would indicate three times as long and so on.
The problem with UTQG Treadwear Grades is that they are open to some interpretation on the part of the tire manufacturer because they are assigned after the tire has only experienced a little treadwear as it runs the test loop. This means that the tire manufacturers need to extrapolate their raw wear data when they are assigning Treadwear Grades, and that their grades can to some extent reflect how conservative or optimistic their marketing department is.
Typically, comparing the Treadwear Grades of tire lines within a single brand is somewhat helpful, while attempting to compare the grades between different brands is not as helpful.
Next is the Traction grade which is based off the tire’s straight line wet traction tested as the tire skids across a specified test surface. The UTQG traction test does not evaluate dry braking, dry cornering, wet cornering, or high speed hydroplaning resistance.
The Traction Grade is determined by installing properly inflated test tires on the instrumented axle of a “skid trailer.” The skid trailer is pulled behind a truck at a constant speed over wet asphalt and wet concrete test surfaces. Its brakes are momentarily locked and the axle sensors measure the tire’s coefficient of friction (braking g forces) as it slides. Since this test evaluates a sliding tire at a constant 65 km/h, it places more emphasis on the tire’s tread compound and less emphasis on its tread design.
The highest grade is AA, followed by A, B, and C.
Finally, there’s the Temperature or resistance grade. This indicates the extent to which heat is generated and/or dissipated by the tire. If the tire is unable to dissipate the heat effectively or if the tire is unable to resist the destructive effects of heat buildup, its ability to run at high speeds is reduced.
The grade is established by measuring a loaded tire’s ability to operate at high speeds without failure by running an inflated test tire against a large diameter high-speed laboratory test wheel. The highest grade is A which can go over 185 km/h followed by B (between 160 to 185 km/h), and then C (between 136 to 185 km/h).
Always remember though that because of a tire’s complex nature and variety of usage, UTQG grades are not reflective of their real world performance.