I have less than 2000 miles on my '21 Lariat and averaging 20-21 combined driving.

 

I think you missed where I said city driving, not average with highway . I live here, and have had multiple F150s and equity partner in biz with a fleet. In city (not overall average with highway, big difference there) our '17 3.5 which was hers was at 11-12 strictly city driving with her back and forth primarily on Federal highway from here to downtown Broward at her office, both stock and tuned was at 11-12. My '19 5.0 Platinum leveled on 33s and tuned, only city driving, 12-14 range. Even few friends with 2.7s are barely better than that. Other F150 friends are basically similar as far as mileage like me, even neighbor. Obviously with highway my average is higher, but with just basically city driving that's what it always comes out to in that range. Totally different vs when I'm out of state even in GA outside of Atlanta, and "town" driving is much better as far as mileage, but that's not even 1/4 realistic to South FL city driving, and when in Atlanta or say Bufford in city and driving, mileage drops like a rock there.

Highway I get 20-21 at 80 hand calc'd on I95. Was just in Keys for mini season. In city I go to LA Fitness m-f and that short drive hurts mileage back and forth as well.
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Look what I posted previously for the Camry rental, rated at 28 city, but strictly city driving here around Laudy, Boca etc, I was at 22.6 avg ( not highway). On highway in that car was damn near at 40 on a long drive lol, that was pretty damn good for a car that size, didn't expect that.

 

Yes, stock tires. I buy all my gas at the same place since I get 10 cents off a gallon. Important with the SOB oil companies screwing us at this time. My Pacifica with a bigger engine gets 28-30 on a long highway run so it should be good gas. It has moved up to 17 around my small town of 10,000 but have not needed a highway road trip recently. I have seen those nuts that stop at red lights in just a few feet when going fast. Almost make me wonder why I don't hear the tires make noise. Tires are at what Ford says they should be for pressure. I don't floor it like most others do in the other lane when on a four or five lane road. Most of them are long gone when I get up to the speed limit. Big giggle is when I catch up to them at the next light. First tank full that got 14mpg was from the dealer. Maybe they have some real cheap crummy gas?
I am sure it will improve with time. I guess I am just spoiled with the Chrysler getting such great mileage.

 

I am willing to bet you a tank of gas it had nothing to do with the 'dealer cheap gas'.
Is there such a think as such cheap gas that it will get 10mpg lower than advertised and still run well?

Has there ever been bad gas, a left over myth from the 70's?

 

The dealer having bad gas was to be a slight joke. I guess it did not sink in with everyone.
Bad gas, well, not good enough, yes. My 1963 Corvair would only run on premium. Put regular in and you thought it was going to die on the spot. And regular gas then was better than the regular we have today.

 

Every few months there is a news story here in NJ where a station got water in their tank and warning people. It shouldn't happen, but if the place is getting a delivery during a heavy storm it is possible.

 

Happened at gas station down street a few months ago, nobody realized till there was about 25 vehicles one morning stranded up road. Water got in the station tanks.

 

No, when testing keep things consistent or you will not learn anything, fillup several tanks from the same station, don't switch octane.