My w900L gets about 1.7 to 2.4, on a good day
If I am down hill and empty with a tail wind. Last year for all miles, I averaged 2.17 MPG.
Oh I should say it has a double frame, 4 axles, 148 inch sleeper, 360 gallons of fuel and pulls a 3+3+3 Trail King trailer. Empty weight, 1/2 tanks, 76200 so don't take this as average when looking at trucks
Like GMAN said the great big radiator and hood is gona cut you down, have to look at "Do you want classic or make $$s". 900's are great trucks, have owned several over the years, but have 0 airodynamics.