I would never call a Texan a liar
I averaged 40.5 during a one days shortish non-stop, no traffic drive in NM of 89 miles and I even pointed it out to SWMBO!
Slightly downhill, dunno! Following wind, dunno! As I am not THAT anal about our JGC gas mileage but that is what we both saw. So, it IS a fact and IIRC, we showed a range on the EVIC of over 800 miles at the time.
We regularly average over 31 mpg on road trips and have regularly seen 35mpg steady state at 55ish mph in a lot of situations
. And I'm not talking about short mileages, the EVIC (IIRC) recalcs it over a 300 mile distance. At the end of each drive, I usually find a gas station and top up and write the EVIC mileage on the receipt so it is together with the fill up amount. I also find the EVIC mileage to be extremely reliable when it comes to any calculations.