yes.. the alt takes power to make power, depending on sizes u can lost up to 10 hp with heavy duty alts with big pulles and about 2 mpg, but its usually about 2-3 hp and about .5 mpg depending on wat alt u get
I have a 300A amputator series and I lose (measured) .5 mpg on the way home (no lights) and .85 mpg on the way to work (with light), this is bumpin all the way, lol. Polo..
PS- If you bump loud enough on the e-way you annoy the person in front of you enough for them to switch lanes, Fact!
i have a mountaineer and a 300amp alternator. ive seen a change of between .25 and .5 mpg depending if the lights are on or off. its not really that big of a deal.