Not calling anyone a liar but I have hand calculated EVERY tank on my 2015 3500 RAM Dually 4x4 Aisin with 4.10's.
No way in HE!! will that truck ever see 20mpg unless MAYBE it's on perfectly flat ground no wind at 55mph.
Towing a combined 32,500# from NW WA to SoCal I averaged 8.6, the first two tanks were really bad with stop and go traffic and head winds. The other tanks conditions were normal. It would have been 9mpg if the first two tanks were normal.
Solo trip from Palm Springs to Phoenix and back driving 70-75 hand calc 15.
Cruising on flat ground no wind computer says 18 @ 60 and 16 @ 70. That is without any stop and go, also the computer reads .4 mpg high.
Driving around the PalmSprings area some freeway some stop and go some roads at 40-50 honest hand calc average 13.
For those wondering that is REALITY!!!