Yup - but only if you use it; there's very little parasitic drag from the
belt turning freely without the AC on.  Years ago someone used to sell a
vacuum switch that would cut out the AC when the engine was under load so
Jim's trick of turning it off when accellerating was done automatically.  If
failing memory serves it just switched the wire going to the AC clutch on
the compressor . .  . . but *most * important to remember is that the AC
should be cycled about once a week to keep the pump oil that floats around
with the Freon stirred up.  Newer vehicles do this semi-automatically for
you by putting the ' AC on' cycles between the 'off' & 'vent' positions of
the air flow control so that you cycle it for a few revs without being aware
of it.  Keeps the compressor from getting jerked on with no oil where it
should be.

Austin