A very good link to wheel and tire dimensions: http://www.rimsntires.com/specs.jsp

And this spreadsheet:  Wheel fitment chart.xls

Year/Model

 Original Tire Size Wheel Size Back Spacing mm/inch Offset mm/inch   Spacers Part numbers and information

Front Rear Front Rear Front Rear Front Rear    

928 1978-1982

 215/60 VR 15 215/60 VR 15 7 J x 15H 7 J x 15H 153.9 / 6.06" 153.9 / 6.06" 65 / 2.56" 65 / 2.56"   Phone Dial type

928 1982-1986

 225/50 VR 16 225/50 VR 16 7 J x 16 7 J x 16 153.9 / 6.06 153.9 / 6.06" 65 / 2.56" 65 / 2.56" 21 mm optional Phone Dial type. 52.3mm offset requires 1/4"spacer, still outside fender

928S4 1987-1991

 225/50 VR 16 245/45 VR 16 7 J x 16H2 8 J x 16H2 153.9 / 6.06 153.9 / 6.06" 65 / 2.56" 52.3 / 2.09" Flat Disc type F: 928.362.14.20 R: 928.361.16.20

928S4 Club Sport

 225/50 VR 16 245/45 VR 16 8 J x 16H2 9 J x 16H2 161.6 / 6.36   60 / 2.36" 60 / 2.36" 17 mm Club Sport type
928 GT 1990 225/50 ZR 16 245/45 ZR 16 7.5 J x 16 9 J x 16 65 / 2.56" 52 / 2.09

928 GT 1990-1991

 225/50 ZR 16 245/45 ZR 16 8 J x 16H2 9 J x 16H2 153.9 / 6.06   65 / 2.56" 52.3 / 2.09"    

928 GTS 1992-1995

 225/45 ZR 17 255/40 ZR 17 7.5 J x 17 9 J x 17 153.9 / 6.06   65 / 2.56" 55 / 2.17" 35 mm 993.362.124.05 992 cup style
Specials / experience
928 GTS 1992-1995     8 J x 17       70   35 mm 993.362.126.01 used on 96-up Targa and 98ish Carrera RS
928 GTS 7,5J x 17 8J x 17 65 52 96536212600 and 96536215405 (Porsche Cup with RDK)
928 S4 225/45 ZR 17 255/40 ZR 17 7 J x 17H2 9 J x 17H2     55 55 20 mm 993 rims are perfect match. But the geometry is basically wrong for 928
928 S4 225/45 ZR 17 255/40 ZR 17 7 J x 17H2 9 J x 17H2     55 70 21 mm 993 rims are perfect match. But the geometry is basically wrong for 928

928S4 1988

 225/45 VR 17 255/40 VR 17 7 J x 17H2 9 J x 17H2     55 55   Boxster style? More bump due to incorrect front offset (63)
90 GT 235/45 ZR 17 265/40 ZR 17 8 J x 17 10 J x 17     55 55   ATP Spa, tyre is 642 mm height, perfect match
GTS 235/45 ZR 17 275/40 ZR 17 8 J x 17 9 J x 17     65 52    
90 GT 255/40 ZR 17 265/40 ZR 17 9 J x 17 10 J x 17           Kinesis setup 928 Specialist sells.
928s4 1988 225/30 ZR 18 285/30 ZR 18 8J x 18 10 J x 18 57 58 997 style. Rears should fit ok, fronts are close but not perfect
89and 90GT or up 225/40 ZR 18 265/35 ZR 18 8,5 J x 18 10 J x 18     50 55   Other 928 require fender mods, all are REALLY tight. Good schocks req