I'd tend to go against the defrost argument...
I do the same thing... Also in the terrible Chi-town winters...
Anyway, have not noticed any considerable/if at all decrease in gas mileage &/or performance!!!

 

update: looks like it might have either been the psycho cold weather of a few weeks back (it has warmed up considerably since then), or it was my air filter. Either way, I'm back up to 260 before the light comes on.

 

I was just on vacation for a couple weeks. During that time, I only got 190 miles before the light came on. I'm too used to the kind of mileage I get on the highway.

 

not sure if this came up yet, but you may need to change the air in your tires from summer air to winter...(it is allot colder in illinois)

 

Another thing that may be causing the loss in gas milage is the fact that you are probably warming up your car. That idle running will burn up a nice amount of gas.

 

well tht and how do u drive...do u smash on the gas or wht...cuz tht does matter as well.

 

The defroster using the AC compressor now makes sense to me. Now I know why my idle drops a little when I have my defroster on full.