..almost forgot, how old is your t-stat and is the temp guage reading normally? Often when t-stats fail in 850s, they stick opened and the engine runs cooler than normal. That in itself will lower mileage. Use of a scan tool will also monitor the engines actual temp which should be at least 93C degrees when warmed up. T-stats should be replaced every 70K miles and often don't last too much longer than that w/o causing problems.