I am the proud owner of a set of new to me CCW 505a's.

Only problem is that I am not sure of the specifications for the wheels I purchased. They came off another C5 and all indications lead me to believe they were ordered specifically for that C5. First order of business is to figure out what tires to mount on the wheels.

I did some research on how to measure the wheels and then measured them. I believe the sizes are as follows:

Fronts
19 Diameter
10 Wide
54 Offset

Rears
20 Diameter
11 Wide
54 Offset

Current Tires mounted to the rims are:
FR 285/30/19
RR 295/30/20

There does not seem to be any issues with the current setup as I have installed them on my car and driven it a bit. However, given the back tires are pretty much slicks now and the front are nankang tires, which seem to be the absolute cheapest tires you can buy, I do not have much confidence in the prior owner's tire choices. Thus, I embarked on my current mission of trying to determine the optimum tire sizes for these wheels.

I found a rim size calculator on another thread here which I have found invaluable: http://www.rimsntires.com/specspro.jsp

When entering the information for the OEM wheel I used the OEM tire sizes for the coupe but the OEM rim width and offset for the Z06. I did this because the Z06 tire size seems to have a bigger circumstance from factory than the coupe. If I used the Z06 circumference the speedometer would have to be adjusted. As the Z06 wheel width and offset are known good dimensions for my coupe I used them because of the larger Backspace dimensions. (Please Confirm whether this makes sense)

With regard to the Rear tires, I think the current size of 295/30R20 works perfect. The outside of the tire falls directly under the edge of the fender and looks really good. I believe I could move up to a 305 but it would stick out further which I don't feel would look good. Also, based on the calculator the backspace and circumference are spot on:


Now for my main dilemma. I am torn between 285/30R19's or 275/30R19's in the front. As I said, the current set up seems to work well but when entered into the calculator it is quite a deviation from the oem setup. Particularly when it comes to the width and backspace:




The 275/3R19's seem to be a bit closer to the oem setup:



So, I would like to go with 275's up front because they seem to be more available and are closer to stock specifications. However, I am concerned because the Nankang 285's sidewall sticks out beyond the wheel only a fraction of an inch. I believe tires from different manufacturer's differ in this regard so it seems a 275 from another manufacture could have a sidewall that sticks out a bit more than the nankangs or be roughly the same.

Does my logic so far seem correct? Is there anything else I should consider? I am thinking of going with Michellin Pilot Super Sports and they don't seem to come in 285's.

Any thoughts or comments would be greatly appreciated, the only request I have is this thread not become a discussion on big wheels and unsprung weight. I understand the larger wheels may affect performance but I had an opportunity to get them for a good deal so i pulled the trigger.

 

Also, I know the wheels need polished and car needs to be lowered a bit. All with due time...

 

Too much info to absorb now.

With that said, the rear tire circumference needs to be 1/2 inch to 1 1/2 inches larger that the fronts or your Active Handling cannot handle it. If you don't care about AH then there are no restrictions. Normal braking and ABS will not be affected if AH is disabled.

 

My opinion without going into the technical aspects are that the smaller tire in front will always look better in comparison to what ever size rear tire you run, plus the steering will be less susceptible to pulling from the ruts in the roads. Also, I'm sure the performance of the Michelin, or any other higher performance tire, will certainly exceed that of the Nankang, even in the smaller size.