I'm using an Android app called DriverDiary to log my expenses on my civvy 85 C30 Crew Cab 2wd Dually. The truck has the 6.2L out of a M1009 in it, 4.10 gears, TH400, and 235/80R16 tires. The vacuum pump on the truck seems to be out because it only shifts at 20 mph and 35 mph, and it doesn't down shift to pull hills. I bought it 8 days ago and have already logged 764 miles on the truck.
Empty, unloaded, I averaged 14.715 MPG. Once I get the new alternator I ordered so I quit breaking belts, I have. 450 mile trip ahead of me with the flat bed fully loaded and a 28' trailer frame, 16 2"x6"s (deck material for trailer), and a 14 bolt rear end on my 1,700 lb car trailer. I am curious what kind if mileage I'll get with the load.
If I am getting nearly 15 mpg with a transmission that high revs every time it shifts, up hill (IL to OH), with a strong head wind... I can believe the 20+ mpg claims.
I am looking to swap in my 3.73's and my 700R4 soon, and considering 37's when I convert it to 4x4 as I prep the chassis and drive train for an Urban Gorilla body swap down the road (staying with dually and flat bed, changing cab and front clip only).
EDIT: Okay, loaded trip from Ohio to Illinois complete. Averaged 11.285 MPG.