[jessaustin]

I agree with siblings that we're not talking about a significant effect on mileage, but technically there is still an effect. When the ventilation fan kicks on, this will be an additional current draw. Since the voltage regulator is an analog device, it will respond to the resulting slight dip in system voltage by increasing the alternator field strength slightly. The engine will then have to apply slightly more torque to the alternator, which will burn slightly more fuel.

Since engine speeds vary greatly, having a dumb alternator that provided power scaled in proportion to RPM would be a disaster.