I agree that winter has an affect on fuel economy and I do see my fuel economy rising now that the temps are above freezing. Problem is, I did the tune up (K&N air filter and plugs) last May and watched the economy go down in late summer/early fall - well before winter. Then it went down slightly more in the winter.

I have noticed that when I do an ECU re-learn (aka, pull the battery cables), my fuel mileage goes back up, but then back down again after one tank. No CEL has come up, but I haven't check for error codes. I'm wondering if it isn't the O2 sensors, but I haven't got around to checking them.