Welcome to the site..
What truck style are you concidering?
Crew Cab, Super Cab, 2x4, 4x4, long/short bedect..
My 05 C/C 4x4 S/B runs the Diablo Predator, and the mileages running mostly empty are about 14ish around town and about 17 out on the highway.. These trucks move through the air like a brick (but extremely comfy
and if you keep the speed to about 65mph or less you may squeak a 'tad' more out of a gallon.. Upwards of 80mph and the numbers drop off or at least seems to.. Also things to concider are any load carried, altitudes, cold area idle times, summer A/C useect. or anything that can offset mileage numbers.. Although it seems that most owners have complained that MPG has 'drifted' a bit since the ULSD switch and not manufacture defined...