Alternator not charging at idle?

[Number21]

In my '73 I can tell how fast I'm going by how bright the headlights are. I checked the voltage at the battery at idle with the headlights on and it's only 12.5 volts. I just added a new wire from the alternator to the battery. Voltage goes up to over 14 if I rev it up any.

Why is the voltage so low at idle? Is it because my engine simply isn't spinning fast enough, should I get a smaller pulley for the alternator? Could this be a bad voltage sense wire on the alternator? The wiring in my truck is pretty hacked up....