Here are all the ZL1 20" wheel and center cap part numbers:
RSK 20x10 black 10-spoke front wheel. p/n 22798741
RSK 20x11 black 10-spoke back wheel. p/n 22798743
Black center cap for black 10-spoke 20" wheel. p/n 22775906
RTQ 20x10 bright 5-spoke front wheel. p/n 9599037
RTQ 20x11 bright 5-spoke rear wheel. p/n 9599038
Silver center cap for bright 5-spoke 20" wheels. p/n 92202223