PART 2: HOW TO READ A TIRE SIDEWALL
At first glance, much of a tire's sidewall seems indecipherable. While enthusiasts generally know the speed-rating letters and tire sizes, there's lots more useful information on there. Let's dive in.
Really? Putting tire age first, before all the juicy stuff? Yup. That's because there are Corvettes out there driving around on age-unknown tires. And if you're unlucky, that rolling time bomb could blow.
There's an easy way to figure out your tires' born-on date: Look for the Tire Identification Number, which starts with "DOT" and has 10-12 letters and numbers. The date code is at the end of that string of numbers. (If you see what appears to be a shortened TIN starting with "DOT" but no date code, check the opposite sidewall.)
Tires produced in the year 2000 or later use a four-digit code: two digits for the production week, and the following two digits for the production year.
Tires made before the year 2000 use a similar code in the Tire Identification Number. However, only three code numbers are used at the end: two digits for the production week, followed by a single digit for the production year in that decade.
While a general rule is to replace your tires after six years, you may choose to keep them on a little longer under certain circumstances. This applies if you've kept them properly inflated, aligned, and out of the sun; if they haven't been cut or punctured; and if they still have more than 4⁄32-inch of tread depth left.
However, 10-year-old or older tires should always be changed, even if they look fine.
The tire's size is represented by a series of letters and numbers. For example, the 2014 Stingray's tires are P285/35ZR19 (99Y). Here's what that means:
P stands for "P-metric" and is used for passenger vehicles. (Other letter designations of less concern to Vette owners: LT for light truck metric and T for temporary spare; no letter means the tire was engineered for Euro-metric standards.)
285 denotes the three-digit section width. This tire is 285 millimeters across from the widest point of the outer sidewall to the widest point of the inner sidewall, as mounted on a specified-width wheel. As we 'mericans prefer to measure in inches, divide the section width by 25.4, and that 285mm tire is 11.22 inches wide.
35 refers to the two-digit sidewall-aspect ratio, or profile. That means the sidewall height is 35 percent of the section width. You can do the sidewall-height math yourself by converting 285 millimeters to inches (11.22) and multiplying it by 35 percent (0.35), which yields a sidewall 3.92 inches high.
19 denotes the two-digit wheel diameter, in inches, that the tire will fit.
The ZR lettering in the Stingray tire throws us a curve. We'll cover speed ratings below, but for now, let's pretend it isn't ZR/Y speed rated, and that it reads only P285/35R19.
R denotes the tire's internal construction. R is for radial, meaning that the tire's body plies radiate out from the center of the tire. Radials represent nearly all of the tires sold today. (There's also D for Diagonal bias-ply construction, and B for Belted construction.) Additionally, an F can follow the R in the description; this means it's a radial run-flat tire.
Load Index/Speed Rating
The three digits to the right of the tire size tell us two things: the Stingray tire's Load Index, and its Speed Rating.
99 is the tire's Load Index, or its load-carrying capability. Passenger-car tires typically use load indexes between 71 (761 pounds) and 110 (2,337 pounds).
Our Stingray tire's load index is 99, which equates to 1,709 pounds per tire, or 6,836 pounds total.
ZR19 99Y—specifically, ZR (Y)—is the tire's Speed Rating. Tires get their speed ratings in lab tests: A test tire is held against a large metal drum under load, and the speed is increased in 6-mph increments until the target speed is hit.
|L||75 mph||M||81 mph|
|N||87 mph||P||93 mph|
|Q||99 mph||R||106 mph|
|S||112 mph||T||118 mph|
|U||124 mph||H||130 mph|
|V||149 mph||W||168 mph|
|Y||186 mph||Z||149 mph*|
|*Note: Only Z speed-rated tires include the speed rating inside the tire size.|
Back in the pre-C4 days, the speed-rating folks figured that the Z's vague, 149-plus-mph rating would be more than enough for street-driven vehicles. Boy, were they wrong.
Tires are still produced with the Z rating in the tire size area, as evidenced by our Stingray's tire. But as you can see, the more specific Y rating is listed after the size section. Here's how it breaks down:
Z indicates the 149-plus mph rating. If the Y were not in parentheses, it would mean that this tire was additionally rated up to 186 mph. But because the Y is in parentheses, it means that the Stingray's tire is additionally rated in excess of 186 mph. Pretty cool, right?
Now that you know about Speed Ratings, it's critical you also understand that only pristine tires can safely attain the rated speed. Any tire damage—as well as tires that are old, overloaded, or under-inflated—can negate the speed rating and may put you and your Corvette at risk. In short: If you're unsure, play it safe and keep speeds down. You only drive the limit anyway, right?