Looking for 'Real facts' answer to mileage question.

Can't really answer the question of how your truck will do but the real world experience with my 09 F-250 with the V-10, 4.10 rear and 5 spd auto is that I get slightly better mileage with tow/haul off. The answer will vary depending on transmission and rear gear coupled with motor size. In my case the motor size and rear gear easily allow the truck to get to OD sooner and hold it comfortably without tow/haul on.