My 3500 is now slightly over 26,000 miles wise and about 26 months old. Since new I guess I have never been under 2,500 or over about 7,500 feet. I just got back from our spring trip to take the 5er into the mountains of southern New Mexico for the summer. It looks like the trip over with the 5er averaged 9.8 miles per gallon, the trip back looks like 17.6 MPG. It got me to thinking if there is any affect on mileage caused by altitude along. If I were to drive 50 miles at sea level and 50 miles at say 6,000 feet fairly level the whole way, if everything were the same would the altitude along have any affect? With the turbo is the difference in altitude compensated for? Just off the top of the head it would appear that if the truck runs at the same level of boost at sea level or altitude the amount of oxygen would be the same and the rate of power production would be the same with the same mileage.

So anyone have real world experience with maybe more then one tank of fuel?

OK, I guess I just asked this question incorrectly so I'll try again. If I run WOT at sea level will my mileage be better or worst then if I run WOT at 6,000 feet?

I couldn't tell you for sure but I would think it would have more an effect on a non-turbo/supercharged vehicle?

Not sure where you climbed from but the 9 something sounds not bad. Sliding back down to the flats sounds about average.
Calculate the whole trip. Use pencil on total miles and total fuel purchased.
I run I-15 or I-25, south in fall, north in spring and the long trip (2000 miles) averages what you averaged on your round trip.
The truck gets terrible mileage on the first and second tank of a heavy trailer pull, then improves as it learns to do some work.
The altitude doesn't seem to be a problem, the turbo gives it the air.
Rolling hills are usually not a problem, WOT up, throttle close down, average same a on flat land.