On a truck forum I like to visit from time to time, there are always numerous threads about gas mileage, and how someone had added 40% better fuel mileage and the ability to smoke the tires in every gear but OD by having a "tune". The the argument begins over who sells the best "tune" (non musical). Guys will swear up and down, take cell pics of the mpg monitor on their dashboard, etc to prove their 2500 Dodge gasser knocks down 22mpg pulling a toy hauler thru Tennessee. Then the Chevy fanboy comes in and states his 6.0 and 4.10 gear gets 18 mpg with 4k of brick in the bed.
No matter what they say, that annoying thing called physics just won't go away.