Check out the truck in my signature. 4.10's, never drops BELOW 17 even when I'm towing. Normally huvers around 19 just daily driving. It's totally possible. Your father in-law must not be a diesel guy.
With 33 to 35" tires, it is very possible. With stock tires, it will be from 14-16 mpg. So, definitely check the tire size. If he's got 285's or 315's on it, then he's probably right on with that mileage.
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