good thing you mentioned something. LS1 SWAP!! GAIN 15 MPG!!!
sliver ls1 got the same mpg (28) going to the lsx shootout with a torquer cam and 4.56 gears.
and to the OP, your driving style and where you drove to most likely is what changed your mileage. Unless you drove to the exact same places, in the same exact weather, with the same amount of acceleration.