I suck at these, but I'll throw a few out, just because I'm bored...
NOSMOKN - "No Smoking" (if you have a clean interior)
SLORYD - "Slow Ride"
OLDPNY or OLDPONY
DONUTS - Use this, and cops will follow you everywhere.

OLD-FE - "Old Iron" (only science nerds will probably get this)
CARBED or CARBFED
POKEY - Because Gumby's companion was a horse, too.
TRIGGR - Yes, another horse name.
GOTBALZ - "Got Balls?"
GOTTORK or GOTTRQ - "Got Torque?"
NOTAGMP - "Not a Gimp"
4-EYES or FOREYE
ITGOES
FOXBDY - "Fox Body"
OLDFOX
4-IFOX or 4IDFOX / 4-IDFOX - "Four-eye Fox" or "Four-eyed Fox"
NOTHEMI - "Not Hemi"
STNGNT - "Stangnet" (assuming that isn't already taken or otherwise copyrighted)
That's all I can come up with on a whim...