Running rich?

[msimasek]

i drive an 02 spyder gs, stock except for a k&n typhoon short ram air intake, installed back in june in time for summer. gas mileage jumped to around 27 with mixed driving.

around early september, still before temperatures really dropped here in PA, my gas mileage plummeted to around 20mpg all of a sudden. its gotten progressively worse since then, down to about 17.5

O2 sensor is just a few months old, plugs are good

is this a sign of a larger problem?