I also have a 5.0 with 3.31s and a manual. While not meant for fuel efficiency, the new 5.0 V8s actually do rather well in that area for the performance they deliver.
These cars will get their best possible fuel economy rolling along in 6th on relatively flat ground at the lowest speed you can get away with being in 6th at. For a 5.0 with 3.31s and a manual, that would be about 40 mph, and at that speed, it could probably do 35+ mpg. This is mostly theoretical, as that isn't a realistic driving environment, but if you ever where somewhere where you were briefly rolling at 40 in 6th, that would probably be the instantaneous number.
I drive mostly on the highway (70 interstate, 60 two-lane roads---yes, the speed limits suck in Wisconsin) and get 29-30 most tanks with ethanol free gas. E10 reduces that by about 2 mpg. The best it's ever done is 31.0 on a tank over 400+ miles.
It does drop off when having fun, but still not as much as you would think. This summer, I drove 150 miles round trip to an autocross, did 7 runs, and all together still averaged 26.5.
