My personal preference for a 9C1 is 15x7 with 3 3/4" bs on the front and 15x8 with 4" bs on back with 255/60r15's all the way around. The 8's on the back look perfect but stick out in front and the 7's look perfect up front but are too deep set in back. If you're running 17's on a 9C1 though on stock Impala SS rims, then a 255/50-275/50 combo looks right. I'm not sure if that would work with the RMS skirts though.
1968 Camaro -my first car.
1995 Purple Pearl B4U Caprice
1995 Black/White 9C1 Caprice--NADACOP
1995 Purple Pearl L99 Caprice
2012 Alto Gray Metallic 9C1 Caprice--The HoDag-Solo axle back exhaust, improved racing OTR CAI intake.