I have a 2006 Dodge Ram Quadcab 4x4 with the Hemi. I get about 14-15 mpg when I'm on the throttle. About 15-17 if I drive politely. I have dual exhaust, no other modifications.
8.6 sounds really low
I have a 2005 dodge ram 2500 with quad cab and 8ft bed. i get 15-16 when on the gas heavy and about 16-18 if i let up some. 8.6 sounds like it needs a serious tune up (plugs,wires, tranny fluid change,transfer case fluid change etc...). also the berings on the front hubs could be bad, that will couse a big drop in gas mileage. a cold air intake will help about 1 mile or so per gallon and if you put a free flowing exhaust on it it will give about 1 more additional mile per gallon.
hope this helps!!!