A few months back, I posted a thread about my a/c compressor making a whinning sound when engaged. Well it got bad enough that I took it into the dealer. They said it was on it's way out, and replaced it with a new one (under my PremiumCare ESP). Well since then I've consistently gotten better fuel economy. At first, I thought maybe it was just a fluke. But I've had 6 tank fulls of gas, and each one of them have improved (vs before the a/c compressor was replaced). My thought is that the old compressor was creating excessive drag on the motor, thus reducing my MPG's.

And yes, my driving habits have remained EXACTLY the same. Same routes, same a/c usage, same tires, same gas, same everything. Anyway, here's an Excel log of my fuel economy that I started some time back.

 

Sounds to me like the AC Compressor was dragging the fuel mileage down. Good catch!