I don't know about you guys, but my driving is mostly 10-15 minute city trips. In the summer I'm up to full operating temp in a few minutes, but in the winter I may go ten minutes before reaching temp. I've always attributed most of the mileage drop to that. When I do long drives where my car is up to temp the difference is not as severe.
My miata has dropped 3-4mpg so far, but that might be due to all the wheel spinning action.
