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Read a recent test of a Pratt and Miller C6 super vette and the wheels did not have the tire pressure sensors (Vette Magazine?) and they had problems getting it to run on the track with the the traction control coming on. Apparently on the C6 the TPS work in concert with the traction control.
Don't know if that is the issue on your C5. I went to a Z06 style tire and wheel combo on my regular C5 and installed TPS. No issues with the traction control.
Did you also alter the size (sidewall depth or hieght I guess) so that the end result was the same outside diameter as the old set up? Larger rims with lower profile tires.
I know traction control/active handeling are programmed with the stock sizes so if you put 18's all around it thinks the rears are slipping and causes problems.
I see you kept the difference (two sizes) but think the overall diameter may be your problem. The wheel speed sensor just knows how may times it turning, not how big it is, so you would think you would be ok, but I''d look at the outside diameter as a start.
You probably don't have a large enough difference between the rear and front tire diameters. On C5's the rear tire diameter should be ~26.5 inches while the front should be ~25.5 or a difference of about 4%. A lot of people have screwed themselves because they didn't pay attention to tire diameters when plus sizing tires/wheels. The EBCM is programmed to expect those differences and nobody but GM can change that programming.