There is, without question or shadow of a doubt, an entirely bewildering set of numbers and notations stamped into the sidewalls of modern tires. Sizing figures arguably the most common set of digits examined on a tire, but there are other tidbits of important information to glean if ya know how to decipher the code.
Let’s start with sizing, since getting correctly sized tires is as much a matter of safety as it is performance. On this continent, customers are generally faced with information which looks like the following: 275/65R20. That may seem a trifle confusing but relax – it’s much worse than it seems. Before we break down what each set of digits denote, you gotta know there are three different measuring standards across those three different groups of numbers.
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We’ll start with the easy stuff. Those final two numerals – 20, in this case – always refer to the wheel diameter in inches. Our example tire is meant to be installed on a twenty-inch wheel. Here, the letter ‘R’ stands for radial, though some manufacturers and humans insist it stands for radius or rim.
The first three digits, 275 in this case, refer to the width of a tire in millimetres. Yes, dear reader, the scourge of metric appears on the side of just about every single tire in our land. Alas. Since this number denotes the tire’s width from shoulder to shoulder of its tread pattern, a higher figure means a wider tire. One with 225 is narrower than one listed as a 235, for example. And, yes, they generally all end with the number five. Go figure.
Next, we find two digits, shown as 65 in our example. This is called the aspect ratio and refers to the measurement of the tire’s sidewall from the edge of the metal wheel to the top of the tire tread. It is a percentage of the tread width. The digits in our example tire size mean the sidewall height is 65 percent of the tire’s 275-millimetre tread width.
Why is this important? Consider our sample tire against one measuring 285/65R20. While both have a sixty-five percent aspect ratio, the second tire will be taller overall since 65 percent of 285 is slightly more than the 65 percent of 275. This could be enough to cause clearance issues against the suspension or bodywork of your car; at the very least, it will throw off the speedometer in yer car since it takes longer for a taller tire to make a full rotation. An indicated 60 mph may actually be 59 mph, for example.
Another set of four digits which are important to know is the so-called ‘date code’ of a tire. This notation is always four digits and generally contained in a small oval stamped somewhere on the tire sidewall, sometimes on the opposite side from the sizing. The first two digits of the date code refer to the week in a calendar year while the last two refer to the year itself. Together, they help customers figure out when a tire was manufactured.
For example, a date code of 1424 means a tire was produced in the 14th week of 2024. This can be useful to know when buying a second-hand car whose seller is asserting has brand-new tires. If a cursory inspection of the rubber reveals a date code of 0319, which means the tires were produced way back in the third week of 2019, you know they’re not being entirely truthful (or are misinformed themselves). Use this knowledge to either leverage the purchase price or at least educate the seller about what they have on their hands. Reputable companies like Sailun always have a date code on its tires.
Speed ratings used to be a big deal in the ‘80s and ‘90s but the democratization of performance has largely relegated those notations to history. When even a four-door Camry can accelerate to 60 mph in less than six seconds, just about all tires need to be engineered to withstand a decent amount of grunt.
Still, it’s worth knowing the scale isn’t linear, with H sitting between U and V with the common perception that H stood for high performance back in the day. An ‘H’ rated tire is good for speeds of 130 mph, while V can handle 149 mph and Z is “149+” miles per hour. In subsequent years, W and Y were used to match the higher speeds of exotic sports cars. Typically, a high speed rating assures great grip and stopping power but may wear more quickly and have a shorter tread life.
[Images: Astrid Gast/Shutterstock.com; Manufacturer]
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Source: The Truth About Cars
