It's been a couple of weeks now with the new Overland V8 and I've got a solid handle on the MPG for anyone who's interested.
6 Hours of driving at 75 mph average using cruise control = 22.7 MPG
Return trip at same average speed = 23.1 MPG
Average around town MPG 16.7 MPG after 3 days.
Highest MPG was on a 4 hour drive at 69 MPH average = 24 MPG
Worst MPG over 5 hour drive was 19 mpg no cruise control and some driving in the mid 80 MPG range. The V8 feels like it's sleeping at 90 MPG.
Speeds and averages were confirmed using a Garmin handheld which never varied from what the Overland's NAV was saying.
I've found these reports from the Jeeps system to be reasonably accurate based on the amount of fuel I put in. My friend's Overland with the V6 does not do much better, but he admits he pushes the engine harder because he's used to the V8 power in his previous GC. He says he has done 26 MPG on a long trip, two tanks. The V6 will work harder than the V8 depending on the driver, so to get the MPG benefit means paying attention to how you drive. For a majority of drivers of think the MPG difference is fairly small.
If you leave the Fuel Economy gauge active over a week or two you start to get a feel of how to maximize efficiency by using the Jeep's considerable momentum to save fuel. Doing so has not effected my arrival times by more than a minute or two!