min sidewall IMHO is about 4-4.5" for it to look right. my '11 charger RT's 255/45R20's are 4.5" sidewall. the 255/50R17's I ran on my old 5th ave were 5" sidewall. big issue is lack of selection in 15's anymore. you basically have BFG radial T/A's, cooper cobras, or the mastercraft labeled cobras, and size for size they're more expensive. the tires I'm leaning towards for my duster if I go for 17's (Nitto NT555's in 235/45R17 in the front and 255/50R17 in the back) are $652 from tire rack. if I go with 15's and 215/60R15 in front and 255/60R15 in the rear (for similar overall diameters), it's $796 for radial T/A's.
