Zaphod. No specific reason, just been a fan of HG2G since I first saw the Brit series back in '82 (10x better than the movie that came out about the same time I got the X)... and I sure wasn't going to call it Arthur DENT, or FORD Prefect... and Marvin had this pain in the diodes in his left (drivers) side (too much bad juju on those 3), Slartibartfast was too long, and Vogon Prostetnic Jeltz was right out.

Previous vehicles:
Juniper
Thaddeus Kahoutek (a Comet, named for a comet)
Star
Jenny
Brunhilde
Lamont (after Lamont Cranston, it was a Shadow) a.k.a "Shitbox from Hell"
Arnold.