[Caddylackn]

My real world day to day experience with the switching to 10% ethanol fuel around October, is I lost about 25 to 30 miles per tank in range. In the summer, I could go about 310 - 315 miles commuting before the fuel light came on, then in winter the fuel light comes on around 280 - 288 miles or so.
This is in my 2006 Crown Vic PI, with a 19.1 gallon tank (fuel light goes on with 16.1 gallons used), which was last year's daily driver. My Crown Vic isn't stock, I have a bunch of performance mods, run a performance tune, and use premium fuel.

So adding 10% ethanol and colder temps = more than 10% loss in gas mileage. Some of this loss is also driving in the rain, which is about 80% of the time around here after October.