just relised something. you turn on the heat, and the ac compressor kicks on. so not only are you reducing your mpg via the blower motor, rear window defoggre, but also the ac compressor. the ac compressor is a big draw on your mpg in the summer, and now the winter time as well.
i noticed my car has a rough idle at lights. today i turned off the fan. poof the no more rough idle. i realize the ac running is supposed to keep the interior dry, but i wonder how much its hurting my mpg? cutting off the compressor would be kinda nice. saving gas would be great!