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.