What MPG is everyone getting with there 6.0's with what done to them?
That's a loaded question. A 2WD truck will get better mileage overall than a 4WD; a stock height truck will do better than a lifted one; and how fast you drive is another factor. With all that said, I'm running a 19.3 mpg average over 110 fill-ups covering 48k miles.
I'm one of the slower drivers out there...seldom run faster than 70 mph; slow roll up to/from stop signs/lights; don't beleive in "rolling coal" and always keep up with my PM's. I think in a couple of hours you'll discover this a "can of worms" type of topic.
I am just looking for a basic answer as in "2005 4x4 whatever programmer this size tire averaging 18-20mpg"
I doubt you'll see a 4WD truck get 20 mpg without keeping the speed below 60mph. There is too much drag in the front axle/transfer case. The type of tire & air pressure your running will also effect the efficiency of the truck. Running street radials at 60+ psi will do better than all terrain at 45-50 psi...that is what I'm doing Michelins street radials at 60psi.
my 06 4x4 mostly stock with stock sized tires and 4" straight exhaust i get about 11 city/ 15 highway. Towed my mud truck last week up to michigan and back, i got 12 mpg. Thats with nearly 10K behind me @ 70 mph!
I am absolutely stock and average about 13 to 15 around town typically. My wife and I recently drove it on a 3300 mile trip (from San Diego area thru Oregon and Washington, east into Idaho, then south back to San Diego area), where I carefully calculated the milage every tank. Before we left I changed the oil (Shell Rotella T), had the transmission fluid changed, changed both fuel filters, and made sure the tires had 50 psi in the front and 55 psi in the rear (tires are stock size BF goodrich). We also ran Stanadyne in every tank. Gas milage (calculated) ran 18.5 mpg to 21.5 mpg, with the average for the whole trip at about 19.5. Also... I was not "easy" on the gas pedal, and in a few spots, I really wasn't easy on it!! By the way, and interesting to me, was the fact that the "gas milage meter" above the windshield consistently read about 0.5 mpg above actual (calculated).
Stock two wheel drive eight ft. bed. 13 to 15 city, 18 to 21 hiway, 14 to 17 mixed. Towing the 31' fifth wheel it gets 11 to 13 depending on hiway and speed. Average 11.6 to the Sierras and 13.2 on return trips. That is all on normal days with no high winds, rain etc. Sierra trips are a 260 plus mile trip each way with no fill ups. When towing I set the cruise at 60 mph where ever possible.
In town, back & forth to work, 13 (at best) - but to qualify that, my trip work is only 3 miles (a tank lasts about 1 month)
We just traveled TX to NC & Back - on the way to NC we towed our 7k# TT and got 12, on the return not towing we got 17 the first 1/2 of the trip and 15 the 2nd 1/2 (30 mph head wind).
I also learned a valuable lesson... when towing, never let the tank get below 1/4 and when the low fuel light comes on... it's to f'ing late! Sat on the side of I10 for an hour waiting on roadside assistance