Newbie Question - Fan

Picked up my Prius II Wendesday. This is my first car with AC (or power doors - or power windows).
Not wanting to compromise gas mileage I proceeded to cool myself in my standard fasion today - by turning on the fan and aiming the vents at my forehead. After about 5 minutes, the fan turned itself off. I turned it on again, had to turn down the air flow to the low setting I had it on, and the same thing happened - about 5 minutes of fan, then it turned itself off again. Anyone know what the deal is?
I tried dialing the set temp up and then down to see if that had anything to do with it. It didn't.
Also, the fuel gauge registered no change for the first 60 miles, then dropped three "points" in the next 80. Is this typical? First tank and all but I'd hate to think I used a third of a tank to drive 140 miles. This is in "ECO" mode only and I drive very conservatively - in all my other cars I was within 2-3 mpg of the EPA estimate. Most of the 80 miles were highway, where I know the car doesn't do as well, but the difference should be pretty minimal. I realize my first tank means very little. Thanks.
Oh and generally the car is nice. Rear visibity is not the greatest but not as bad as one often hears, although I will remove the rear headrests when no one is back there. I appreciate the oversized mirrors. Smooth running car; not a powerhouse but I wasn't expecting that. Adequate for all practical needs; enough merging power. The shifter is annoying with the reverse and drive - reversed.

 

Thanks Tom!

 

Thanks Doc and Pat. My Previuos cars ('81 Escort, '82 Corolla, '95 Tercel, 02 Civic HX (a lemon) also had "guess gauges"; I guess I expected the Prius with its high tech image would have overcome that.
The awkwardness with the shifter, at least for me, is that others I've used didn't utilize an "over and up" or "over and down" motion so having reverse above drive didn't product that counter intuitive feel I get in the Prius.

 

Turning off "auto" didn't fix the fan problem; I was still fighting it with auto off and AC off but when I adjusted the temp down to where it displayed "Lo" rather that a temp everything was fine. I could set the fan at a low speed and it would just stay there, even after parking and re-starting the car.
Guess people were right about the guess gauge. I fuel up at 319 miles. Tank took 7 gallons exactly; computer says I averaged 46.8 mpg. Weather is cool; this morning at start-up was 37 degrees F so for the first few hundred miles I seem to be doing pretty well.