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The higher your mileage, the greater (worse, actually) the effects of your auxiliaries. Your fan, your headlights, they all consume fuel (via the added engine power to drive the alternator harder to produce the extra amps) at a fairly steady rate (when they're on).
My headlights burn roughly a sixth of a horsepower's worth of electricity, I'd guess it's a quarter horse when alternator efficiency and mechanical losses (belt slip and flex) are included...it could be more, but a quarter horse at the crankshaft costs me 2 to 2-1/2 ounces of fuel per hour. At cruise MAX burns about 60 ounces an hour (and gets about 100 mpg) so turning on my headlights knocks me down about 4%, or 4 mpg. So yeah, if you're getting better than 50 mpg normally, turning on your heater fan is going to cost you a mpg or two.
Now if I'm driving my 20 mpg van at 60 mph, I'm burning 3 x 128 fluid ounces of fuel an hour, so its headlights are roughly 1/2% of its fuel consumption, roughly 1/10 of an mpg. Barely detectable. But in MAX, if I'm going 55 with the lights off I get better than 100 mpg and with the lights on, worse than 100 mpg.
The more you improve your mileage, the more you'll notice the little powersuckers.
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