A/C and heater causing mpg to drop to 11mpg from 19mpg...normal?

simple, just push the button

 

Yeah , that's not how it works , if either defrost setting is selected , the AC is on , doesn't matter if you push the AC button or not

 

Sorry for the delay in update, I was camping. Still seeing the same issue.

In the winter I used the defroster pretty often. Sometimes I noticed the A/C light was on after driving a bit and would turn it off and other times I never did (I wasn't paying a whole lot of attention at that point). But that could explain why I was getting up to 14mpg in the winter occasionally and then 11-12mpg on the next tank.

I always thought it was a fluke the A/C went on with my heater, hah! Thanks for explaining that mystery to me. If it wasn't 100 degrees out every day I'd try driving a tank with the heater on and no defroster/A/C to see how that affects the mileage.

I have an AT to answer another question. I'm not noticing any abnormal downshifting when I run the A/C though. It really boggles me how I don't seem to be loosing any power or have any weird engine stuttering.

I tried disconnecting the battery for an hour at the recommendation of the dealer to let the computer reset, didn't seem to change anything though, still horrendous mileage with the A/C on. I mentioned to them I do tow a 3500lb trailer sometimes and they thought the computer might have got stuck in some weird mode. (BTW, I get about 5-6mpg towing the camper with A/C on, used to get closer to 12-13).

Tried making an appointment to get the compressor checked but they can't get me in for 10 days and I have to go out of state to work again on the 24th. Also wanted them to do a full computer diagnostic to check the fuel system and sensors. Might have better luck when I'm in Wyoming with a dealer up there.

I stuck my head in and tried looking around at the compressor and pulley but couldn't see too much (not quite sure what to look for though other than obvious damage). I will definitely replace my thermostat since that is cheap and easy for me to do myself. Try to get it done tonight.

Thanks again for all the input! I was at a dead end and I have some ideas to work off again which is giving me some hope.