My '04 F350 2wd V10 mpg is 100 miles less per tank full when I use my heat/defrost. I moved from a warm climate to colder one but I purchased this truck when new and I don't ever remember a difference when using the heater/defrost. A/C yes. When heater/defrost is on, I do feel the compressor cycle on and off. Is this normal because in previous winters I never noticed the cmpressor or poor mpg when heater is in use. I keep up with regular maintainance and inspect. Recently had plugs changed with 120k on them and they just as good as the new OEM's that went in. Is the Heater and defrost supposed to work this way or am I missing something?

 

The compressor is supposed to operate in the modes that the owner's manual specifies.

Operating the compressor is unlikely to make more than a few percentage points of difference in fuel economy.

 

Thanks. Think I found the answer in your HVAC FAQ's.