Yer going to get worse mileage in the winter in anything you drive. Everything is colder, more resistance in the diffys cuz the oil is thicker (even synthetic), more resistance in your bearing grease, etc, it all adds up to more gas burned.
I get 17-20 on the hwy and as low as 13-14 around town in my 4.4l.

Problem is, these things are sooooo much fun at full throttle...