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You can go 265or 275/ 35/ 18 front and 295 or 305/ 30/ 19 rear. I run 265/35/18 front and 305/30/19 rear and the roll out is the same as a stock C5 ZO6. No TC or ABS issues and no rubbing. Lowered on stock bolts.
I thought that changing tire size does not require a speedo recalibration?
As for the size do a search around C5 general. A lot of true tested combos for 18/19. 30/305 very common for the rear and 265 or 275/35 very common for the front. I run a 275/35/18 in the front and a 295/35/19 in the rear. Looks great with no issues.