After damaging our project C5's wheels in an off-road excursion a few months back, we embarked on a search for a new wheel/tire combo that would improve adhesion while also lending the car a modern, custom look. While we liked the styling of the stock early-C6 wheels, differences in rear offset--79 mm on the C6 wheel, versus 63 mm on the C5 rim--ruled out the possibility of a direct bolt-on. And with all of the performance modifications we've made to the car, spacers didn't seem like a viable option. For help, we contacted our friends at Mid America Motorworks. They suggested a reproduction version of the C6 wheel that would bolt directly onto our C5.
We chose an 18x8.5-inch wheel with a 57mm offset (PN 634-068) for the front, and an 18x10.5-inch unit with a 64mm offset (PN 634-070) for the rear. The wheels are sold in pairs, with the fronts retailing for $499.99 and the rears going for $529.99. Don't let the "reproduction" label fool you: These are high-quality units made from 356 aluminum alloy and constructed with a counter-pressure casting method for maximum strength. Each casting is then CNC-machine-finished, pressure tested, and inspected for lateral runout.
With our wheels picked out, it was time to choose a new set of tires. But before we tell you which set of rubber we settled on, let's take a moment to cover the numerous safety and performance features we considered as part of our selection process. These included size, type, speed rating, treadwear, and load capacity. Most of this information is found in an alphanumeric code on the sidewall of the tire. Let's decipher these letters and numbers, using the P245/40ZR18 93W code found on our new front tires as a sample.
Deciphering Tire Codes
* The P designation indicates this tire is for a passenger car. Other codes include LT (light truck), ST (special trailer), and T (temporary use, such as a space-saver tire).
* The 245 indicates the overall width of the tire in millimeters and is measured from sidewall edge to sidewall edge.
* The 40 represents the aspect ratio, which is the percentage of the sidewall height to the tire's overall width. Tires with lower aspect ratios typically have shorter sidewalls.
* The Z code indicates the tire's speed rating (see chart), while the R refers to its radial construction. Other construction codes include B for bias belt and D for diagonal.
* The 18 represents the wheel diameter, in inches, that the tire will fit.
* The 93 represents the load rating, or the amount of weight a tire can safely carry. Most passenger-car tires have a load rating of between 65 and 109.
The W refers to the maximum velocity the tire can sustain for 10 minutes without failing. This component is only found on tires with a Z speed rating.
There are a few other codes on the tire that you should know, such as the treadwear code, traction grades, and temperature grades.
The treadwear code is a three-digit number located on the sidewall. This is a comparative rating designed to give you an idea of the expected tread life of a tire. These grades are based on actual road miles, in which the test tire is run on a vehicle alongside an identical vehicle fitted with a control tire with a treadwear rating of 100. These vehicles run a total of 7,200 miles on a 400-mile test loop in Texas. Both the test and control tires' wear are measured periodically during and at the conclusion of the test. The tire manufacturers then assign a treadwear grade based on the observed wear rates.
Treadwear ratings typically begin at 100 and increase or decrease in increments of 10, depending on the tire's expected tread life. A rating of 100 represents an expected life of 30,000 miles. Using this system, a tire with a treadwear rating of 150 should have a 50 percent longer tread life, or 15,000 additional miles over the base tire rating.
That's how the system is supposed to work, anyway. But because the Department of Transportation isn't involved in tire testing, the agency depends solely on the manufacturers to test their own tires and report the results. Accordingly, treadwear ratings are open to some degree of interpretation, and can thus be legally manipulated for marketing purposes. Don't be fooled if you see a tire with a treadwear rating that seems extreme. A good rule of thumb is that soft-compound tires--including most high-performance units--will wear faster than hard-compound ones.
The traction grade is based on a tire's straight-line wet performance on a specified test surface. The rating is determined by installing the test tire on a skid trailer. This trailer is then pulled behind a truck at a constant 40 mph over wet asphalt and concrete. The brakes are briefly locked, and sensors measure the tire's braking g-force.
The temperature grade indicates the amount of heat that is generated or dissipated by the tire. If the tire is incapable of dissipating heat effectively, its ability to run at high speeds is reduced. This rating is determined by placing a spinning tire against a large metal drum, creating friction. Sensors then measure the heat generated by the tire both during and after the drum phase. Every tire sold in the United States must be capable of earning a "C" rating, which indicates its ability to withstand 85 mph speeds without failing.
After educating ourselves on tire specifics and looking at several different performance-oriented street-tire options, we decided to utilize the Nitto Invo. We felt the Invo--sized 245/40ZR18 up front and 285/35ZR19 in the rear--would make an ideal complement for the car's existing upgrades and deliver the best possible balance of all-weather performance and ride comfort.
Nitto used state-of-the-art computer engineering to produce a unique tread design that's said to be both aggressive enough for track use and quiet enough for everyday driving. Now that our C5 is back on the road, we'll be putting those claims to the test in the weeks ahead.