I'm also in the Chicago area, and have a short commute (2.9 miles), I average 49 mpg (lifetime, computed), with some work. In the winter I can drop to the upper 30's (I had 36 one time, but that was when the coolant pump was giving problems. That was fixed under a Toyota Service Bulletin, and since then my cold-weather mileage improved). In the summers I get low 50's to mid 50's (depending on how much bike riding I do for the commute, saving the Prius for longer trips). I added an engine block heater, and that added a few mpg to my winter rates, I don't think I dropped below 40 mpg on a tank last winter. I tend to avoid using A/C and heat (unless the defrost is needed), since it takes a couple miles before you really notice the difference anyway, and by then I'm nearly home. I also have a scanguage, so I know when I can truly glide, and my commute is on roads that are generally 30 mph and uncrowded (I usually avoid the 45 mph road, if I get there in 6 minutes vs. 7 minutes it's not a big deal to me). I grill block in the winter, and I have my tires at 40/38. I recently got Michelin Energy Saver A/S tires, that might've added 1 or 2 mpg.