I know this sounds crazy but my gas mileage seems to have dropped since the cold weather is here.
When the heat is on is the a/c compressor engaged ?
I know it is on when the defrost is on.
Could the choke be sticking?
Is it electonic?

 

Cold air is dense air with means that technically more oxygen is going into your engine.
The PCM is programed to try and keep the ideal air fuel ratio of 14.7:1
As a result more oxy means more gas is required....thus poor mileage. so unless your mileage is REALLY out of wack I would say normal.
Also....if something was Wacky...there would be codes set more than likely.

but to answer your question...the ac compressor will be commanded on in the following positions...Max AC, vent, vent/floor, defrost/floor and defrost. Temp setting has no effect.

 

Thanks.
I didn't think abouthe the air density or the mixture.
It had dropped 2-3 mpg when the temp wnt down.

 

Yep. Happens every winter to Me as well. 2-3 mpg drop. It's normal.