I hate to sound like a grinch, but PLEASE do a search before posting. I think there are at least 3+ topics out there in the past few weeks about gas milage getting worse in the winter...here are two:

http://www.xterraownersclub.com/ubb/ultimatebb.php?ubb=get_topic;f=78;t=001062#000000

and

http://www.xterraownersclub.com/ubb/ultimatebb.php?ubb=get_topic;f=78;t=001112#000001

but yes, during the summer I got around 17-18mpg..in winter I now get about 14-15mpg, its the nature of the beast (and the cold)