I just bought a set of C5's off ebay, now I need help picking out the right size tire. They are 17 x 8.5 in front and 18 x 9.5 in back. What size do you recomend? Thanks.

 

You need to match the stock diameter of 25.65" as closely as you can. For a 17x8.5" wheel you would use a 245/45-17 or a 275/40-17. Many manufacturers do not recommend that the 275 be used on an 8.5" wheel, so when you look for tires, also look for the manufacturer's recommended rim width for the specific brand/model tire you are looking at.

For an 18x9.5" wheel the 275/35-18 would be the closest diameter match. You could also consider a 285/35-18 which is only slightly larger than stock, or a 295/35-18 which will be about 1/2" larger than stock, and approaching the size difference where you might want to consider correcting the tire diameter in the PCM. The 295 may also not be recommended with the 9.5" wide rim.