I'm convinced I should run the steelies with snow tires year round. For the past couple of months while running the HX's and summer tires I've been averaging 27MPG (averaged 31-33 this summer). I just got through my first full tank with the steelies and snow tires (Nokian Nordman 4's this year) and I got 29MPG. Same weather, same driving style, same exact route each day, etc.

Just an update to this thread. Back in March, while I was sitting at a red light, my resonator pipe decided to literally fall off of my Civic. My mechanic said it was rotted out and that the head pipe was also rotted so both were replaced. My Flowmaster muffler is only a couple of years old as is the exhaust manifold. So now it has almost an entirely new exhaust, clean AEM air intake, proper tire pressure, I've been told that the plugs/wires are good and I've run fuel system cleaner and Lucas fuel treatment and I'm still only getting 32MPG (not 35MPG like I used to with the steel rims).