I'm thinking you might need to shift sooner and try to keep the RPM's lower. I average in the 19-24mpg range and I generally drive somewhat aggressively. Last time I was on a road course I beat the shit out of my car for a couple hours and still averaged 12mpg.
I'm betting the 4.10's would contribute to the bad mileage too. I'm still running stock gears but will be moving to 4.10's very soon and will know for myself what they do to MPG's.
Most likely in the tune. I'm averaging 9.9 mpg.