100% is an absolute maximum, as is zero an absolute minimum by definition.
however...
The nuvi 660 has an earphone socket on the bottom right side.
Connect a mini headphone jack plug to your in-car stereo system and you can make your neighbours DEAF with your sat/nav (dependent on power of your system)
Good luck.
p.s. i'm going out now to by ear-muffs in anticipation of you blasting/broadcasting 'British English-Daniel' world-wide.
Volume Control
