I pretty much stop listening to people when they claim what mileage they get especially when they are an extreme fanboy of a brand or particular engine because you know they are going to lie. Most only go by the lie-o-meter and if they do tell you highway mileage it is whatever the lie-o-meter stated on a flat road in perfect conditions instead of hand calculating a tank that was mostly highway.
A quick look at places like Fuelly.com can help you decipher who is BS'ing or not. For instance, with a V-10 Excursion, out of 95 people tracking their mileage and over a million miles logged, the average is 10.03 mpg. With the Ram 2014+ 6.4L 2500, there are 209 people tracking with almost three million miles logged, the average is 11.94 mpg. With the post DEF 2014+ Ram CTD 2500, there are 753 people tracking with over 15.5 million miles logged, the average mileage is 15.48.