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Offset is the distance from the wheel mounting flange to the centerline of the rim. backspace is the distance from the mounting flange to the edge of the rim on the inside.
56mm offset is correct for a 9.5" wide wheel. The backspace will be around 7.5" For an 11" wide wheel you can use a 50mm offset to keep the wheel inside the wheelwell. Backspacing on this wheel will be about 8"
The 11 inch wheel was the width of the rear wheel on the ZR-1 and the '96 Grand Sport coupes. The two wheels have different offsets (38mm for the ZR-1 and 50mm for the Grand Sport coupe).
This difference was because the rear portion of the ZR-1 body was wider than the standard C4 coupes; it was 3" wider at the rear end. A "real" ZR-1 wheel will bold on but it will extend out from the wheel well opening. The offset on the 11" GS wheel allowd the wheel to sit in the wheel well but the GS Still had a fender flare to cover the tire tread.
The Grand Sport convertibles used 9.5" wide wheels at all corners.
Having the same size wheels and tires allows you to rotate the tires for better mileage. The 11" ZR-1 and GS wheel used a 315/35 tire and the 9.5" wheel can take either a 275 or 285/40 tire.