The cooler the air and the easier an engine can breathe the better it's efficiency. 20s while cruising at 45. What gear? The 2.4L is torquey so you can be in 5th or 6th at 45 cruising and should be in the 30s... Did you reset your ECU after the install to allow it to learn the new air flow characteristics?
Hey I just installed my TK CAI a couple nights ago and for SOME reason I get ridiculously good numbers for the highway but i get down to 20s just cruising in 45mph zones, im thinking its because of the cold, if so then Ill probably go to an SRI setup for the winter... I dont rev either lol