you obviously speak as a person with no electromechanical theory knowledge.
electrical load directly corelates to mechanical load.
the more electricity used by a vehicle, the more the alternator charges. the more the alt charges, the more of a mechanical load it is.. the more mechanical load, the more fuel used.
Ok rocketscientist.. look at a generator set.. you know.. portable one.. or heck.. even pto.
ever notice they will tell you fuel usage at half load, or full load.
ever notice the fuel usage goes up as load goes up.
simple physics.. no free lunch.. you use more electricity.. you use more fuel.
if you can reduce the incandescent load on a vehicle by replacing all but headlamps out with LED's then you an reduce overall fuel consumption...
sheesh.... this is high school level stuff here people....