From an average of 20.8mpg, unloaded on the highway, to an average of 18.2mpg with the FWC on and loaded up. That's with a 3.4ltr and an auto tranny. So a loss of almost 3 mpg.
In 4wd for a full tank, loaded up, I've seen as low as 15.4mpg.
I tracked my mileage numbers dutifully the first year, not so much the second year when gas settled above $3 a gallon. Too depressing...
FWIW, there has yet to be one night while kicked back in the FWC, dinner cooking, the heat on while it's cold or raining outside that I thought about those lost 3 mpg.