LORELAI: [The Volvo’s] also excellent for cranking Metallica.
RICHARD: Cranking Metallica? … If that’s some sort of drug reference, it isn’t funny.
Lorelai says that Christopher’s Volvo is excellent for “cranking” Metallica, meaning “listening to Metallica loudly on the stereo”. The car really does have a great stereo system, but Lorelai is teasing Christopher, because Metallica is her favourite band, not Christopher’s.