[bertmaster2000]

i thought i would mention that around here when the temps fall below about 20F and stay that way until April (i am in AK), my wife's 2001 outback with 3.0l does the same; goes from 26 mpg to about 20 mpg.

if you also live in a cold environment i think there is nothing wrong with your plugs, every car simply uses more fuel to move all the wheel bearings, gears, etc. when it is cold outside.