2002 chevy Tahoe transfer case and transmission cause my truck to get bad gas mileage?

[copernan]

My truck was getting 12.5 mpg in october of this year, and 1 week into october my transfer case went out and i had a used transfer case put in the tahoe, now i get 9.1 mpg, is there a possibility that the transfer case has screwed up the mileage or does it have to do with the temperature being cold now