It's easy to take tires for granted. We rarely interact with them, and usually only come down to their level when it's time to check and add air. So it's no surprise that some see tires only as rolling rubber hoops, built for laying tracks and doing donuts.
But the truth is, your tires may be the most high-tech component on your Corvette. And just as every new Vette is the pinnacle of user-friendly performance, the latest tires are the most reliable, most comfortable, and highest- performance versions in a long production history.
To drive home how lucky we are right now, put yourself in the shoes of a tire engineer: Ever since Charles Goodyear stumbled upon vulcanized rubber in 1839, engineers have been making trade-offs with tires. There are so many criteria working at cross purposes—traction, comfort, noise, wear, rolling resistance, handling—that compromise has always been a must. Remember those rock-hard "performance" tires that used to shake your fillings out?
But thanks to recent advancements in tire design and production, modern tires are able to lessen those compromises. For example, the 2014 Stingray's Michelin Pilot Super Sport ZPs carry a 300 treadwear rating, higher than many recent Corvette tires. Yet the Stingray pulls more than 1g in lateral acceleration, has less rolling resistance (for improved fuel efficiency), and rides both smoothly and quietly. That, my friends, is called progress.
This article will help you learn more about tires—what they're made of, how they're assembled, how to read their sidewalls, and most importantly, how to take care of them.
PART 1: TIRE CONSTRUCTION
Tires are highly complex, multi-layer constructions that meld specialized rubber compounds with reinforcing materials. Here's what your performance tires are made of:
Tread is what hits the road, and each tire's tiny contact patch keeps your Corvette on it. It's made of a complex tread compound comprising rubber, along with other materials such as silica, carbon black, polymers, and bonding agents. Today's advanced tread compounding can make softer compounds rigid for high-performance applications—so it's no longer accurate to state that all high-grip tires are "soft" compounds, and long-life tires are "hard" compounds.
Tread patterns aren't just a cool design, they play a huge part in a tire's performance. They handle water removal, help handling, and reduce noise. The patterns can be asymmetric (not the same across the tread centerline) or directional (requiring a particular rotation).
A tire's tread incorporates a center rib and tread blocks that assist with dry handling, grooves that evacuate water, sipes that open and lock together to increase handling, and shoulders that can be square or rounded.
Sidewalls help support the load, protect the tire from curbs and other hazards, and, in the case of run-flats, help keep a deflated tire rolling. In addition to the obvious rubber outsides, sidewall internals include fabric carcass plies. They can also be reinforced with fabrics or steel cords to help steering response and cornering stability.
The bead allows the tire to be secured to a wheel. It looks like a smooth, U-shaped circle at the very bottom of a tire's sidewall, and it's internally reinforced by steel wires.
Cap plies—typically used on performance tires—are found right under the tread. These nylon layers control the shape and size of the contact patch during acceleration, braking, and cornering. They can also help smooth out the ride.
Steel belts form a foundation for the tread. Made of high-tensile steel, these belts provide tread stability, and guard against impacts and punctures.
Body plies/carcass plies are fabrics like polyester that help give the tire its shape. Modern body plies run radially at 90 degrees to the tire centerline—hence, the name "radial." In contrast, older or reproduction bias-ply tires run the plies at 45 degrees to the centerline.
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?
UNIFORM TIRE QUALITY GRADE STANDARD: TREADWAR, TRACTION, AND TEMPERATURE
The Uniform Tire Quality Grade Standard, or UTQG, has three main aspects: Treadwear, Traction, and Temperature.
Treadwear Grades are numbers that indicate how long a tire will last in typical use. They're based on how much the tire wears, as compared with a test tire, over 7,200 miles. A treadwear rating of 100 means that the wear rate is equal to the test tire's wear. A rating of 200 means that the wear rate is double the test tire's wear rate, and so on. While it does not suggest a specific mileage range, it's reasonable to infer that the higher the number, the slower the tire will wear.
Treadwear Grade Examples
|300||Goodyear Eagle F1 GS EMT||2005 Coupe|
|300||Michelin Pilot Super Sport ZP||2014 Stingray|
|220||Goodyear Eagle F1 Supercar||2004 Z06|
|80||Michelin Pilot Sport Cup ZP||'13 ZR1 with PDE performance package|
Keep in mind that the DOT doesn't do across-the-board testing on every tire sold. Rather, the various tire manufacturers test their own, or hire companies to test for them. As the testing may vary between companies, the Treadwear Grade numbers can vary between brands. So if you want to figure out which tires will last longer, your best bet is to compare treadwear ratings between different tires from the same company.
Traction Grades are based on how a tire performs in a straight-line wet skid test. The tires are installed on a "skid trailer," which is pulled at 40 mph over wet asphalt and wet concrete surfaces. The brakes are locked, and braking g forces (the tire's coefficient of friction) are measured to determine the Traction Grade.
Traction Grades are AA for the top-performing tires, A, B, and finally C for the lowest-performing tires.
A couple of notes: Today's highest rating, AA, was only added in 1997. And interestingly enough, Traction Grades have nothing to do with dry braking or cornering, wet cornering, or hydroplaning tests.
Temperature Grades indicate how much heat is generated, or dissipated, by a tire. Testing is done by running an inflated test tire against a large-diameter, high-speed laboratory test wheel. The test tire must operate at a certain speed range, without failure, to earn an A, B, or C grade.
Temperature Grade Standards
|A||Over 115 mph|
|B||Between 100-115 mph|
|C||Between 85-100 mph|
|Note: Don't get the ambiguous Temperature Grade A number of "Over 115 mph" confused with the previously mentioned Speed Rating letters, like a 118-mph "T" speed-rated tire. The Temperature Grades cover a tire's heat-generation and -dissipation abilities only.|
Maximum Inflation Pressure
A tire's maximum inflation pressure is the highest "cold" pressure that a tire is designed to contain. It's located in small print near the tire's bead (close to the wheel), and shown in both kPa and psi.
Note that max pressure is very different than recommended pressure (see the "Corvette Tire Care" sidebar for more info on recommended pressure). Running your tires at max pressure will greatly increase treadwear, while negatively affecting both ride quality and handling.
Materials And Layers
This information tells you what the tread plies and sidewall plies are made of.
Near the max pressure text, you'll see text like this:
Tread: 2 Polyester + 1 Polyamide + 2 Steel
Sidewall: 2 Polyester
C7 Tire Development
Every 2014 Stingray wears Michelin Pilot Super Sport ZP high- performance tires. There are two tire choices: The base car gets P245/40ZR18 fronts and P285/35ZR19 rears, while Z51 Corvettes wear lower-profile P245/35ZR19 fronts and P285/30ZR20 rears. Both sets are speed rated to over 186 mph.
Michelin's technical partnership with Corvette started during the 2004 Twelve Hours of Sebring, where the C5.R race cars used Michelins competition tires. In 2007, Michelin was awarded the original-equipment fitment for the upcoming ZR1. Since that time, the Corvette team has collaborated with Michelin to develop ultra-high- performance tires for other Vettes.
While GM was working on the Stingray, Michelin began developing a tire that would match its performance. This was certainly a tall order, as the new Vette's tires had to have good tread life and efficiency, as well as low noise. But they also had to have excellent performance in the rain, and immense dry handling and grip levels for low lap times. Michelin spent almost three years and conducted nearly 12,000 hours of tire design, development, and testing using state-of-the-art computer modeling and thousands of miles of testdriving.
An impressive aspect of the Pilot Super Sport ZP is its immense grip from a 300-treadwear rating. That grip is thanks to an optimized contact-patch design, along with footprint-stress research and precise tire-profile choices. Tread-sculpture stiffness optimization was also key, so Michelin focused on the front/rear balance for wet and dry performance, while delivering very good tire wear.
Different design choices were made for the Base and Z51 tire models, to best match the desires of GM engineers. The base car's tire gives balanced, all-around performance, along with high ride comfort, wet capability, and rolling resistance.
The Z51 package's tire, meanwhile, offers near-racing-slick grip and handling levels. A dual tread compound uses ZR1 tire technology to meet the Corvette team's track-performance goals. Different belt angles and lower, stiffer sidewalls yield even better cornering prowess.
All that work clearly paid off: In GM testing, a Stingray equipped with the Z51 package recorded an amazing 1.03g lateral-acceleration number. How's that for tire performance?
Corvette Tire Care
Corvettes have long been shod with top-notch rubber, but 1990-present models got truly cutting-edge tire technology. In 1994, the Vette was the first vehicle to get optional run-flat tires. And in 1997, Goodyear's Extended Mobility Technology tires became standard fitment. C5 Z06s yanked our heads back with grippy Goodyear F1s, and the C6 ZR1's Michelin Sport Cup gumballs obliterated the line between street and race rubber.
Because our cars come such amazing tires, it just makes sense to keep them in tiptop shape. Here's how:
- Locate your car's proper tire-inflation pressure. It can be found on the tire decal, which is usually mounted on the inside edge of the driver's door or in the glovebox.
- Check your tire pressure once every few months. Tires lose around one pound of pressure a month, and can lose an additional pound for every 10-degree drop in outside temperature.
- Check tire pressure in the early morning, when the temperature is lowest and the tires are "cold."
- Digital or dial-type pressure gauges are the most precise, but a stick type will work too.
- Use an upside-down quarter in the tread grooves to measure your tread depth. If part of Washington's head is always covered by the tread, you have more than 4⁄32-inch left. Tires with less than 4⁄32-inch of tread can affect how your Corvette handles, and should therefore be replaced.
- Use your hands to feel around each tire. Check for uneven wear, bulges, punctures, or indentations. If you find a problem, don't drive the car until you've had the tire inspected.
- Follow the tire-rotation schedule, and always have your car aligned at a reputable shop with experience servicing Corvettes.
- Only use non-petroleum-based products to clean your tires.
- When parking your Corvette for extended periods, be sure to keep your tires properly inflated to avoid flat-spotting. Also keep them out of the sun if possible.