Heater costs 2 mpg?
 

Going into work today I had the heater on and was getting far worse mpg then I was several days ago.
Temps had droped from being in the 60's and 70's to being in the 30's and 40's with a cold front that moved in causing my super awesome tank average of 58 mpg to plummet.

On my trip to work I have several flat stretches so I held the car at 45 mph to get a level mpg reading and then turn on the heat and my mpg would drop by 2 (4% loss).

Even though I did this test 20 times (16 of 20 lost 2 mpg, 4 of 20 lost 1 mpg) I know it's far from being a scientific set of A-B-A tests.

Frostbite is not sexy so I'll still be using the heater, I was just wondering if anyone else noticed lost fuel economy due to turning on their heater.

Heaters are known to use a lot of energy. my small space heater i use in the garage uses 1500 watts the only thing is, i thought the heaters in the car only use a fan because the heat is from the engine coolant?

Thats what I thought also.
My coolant temp was above 180F so it was plenty hot enough.

I did not notice any increased revs like I would with running my A/C but that was almost certainly a drop in mpg.

its the increase of electrical load on the alternator.

My heater fan on the highest setting is drawing over 80 watts, so not quite as much as the head lights but it's getting close, also some cars do turn on the A/C at least for defrost to help dry the air.
Was the drop instant? if it was then I would think that the drop would be from an electrical load, but if there is a lag time then it's from the fact that it's pulling heat from the engine, the jump back up with an electrical load should also be instant where a cooler engine again will have a slight lag time, maybe 30 seconds or so?

Drop only took 1 or 2 seconds I would guess it took about 4-8 seconds to come back up after I turned it off.

Was only turning heater fan to 2.

what were your other electrical loads at the time? do you have a scan gauge or some other way to see what your battery voltage is? I'm guessing that your alternator is either turned off or putting out just an amp or two and when you turn the heater fan on it's energizing the whole alternator, I haven't checked to see what kind of alternator controls your car has but I would guess that is what is happening.
Another way to test is to leave the fan on but to turn the heat down as that should turn the valve to the heater core off.

Modern heater fans draw close to the same wattage whether they're set to their lowest or highest setting. All they do is bridge different resistors at their lower speeds. I'm thinking your AC is running when your heater is, to dehumidify the cabin air. It's not logical for just the heater and fan to draw 2 mpg.

yah, I unclip the lead on the compressor in the winter (and usually leave it unclipped for the summer). My defrost turns on the compressor to dry the air and I usually just need some airflow (a towel and/or fog-x are good options too).

hmm.... will have to figure out how to do that.
Air is dry enough as is during the winter around here.