My point is real world gains don't exist and it's foolish to account for that over gear ratios within the transmission, axles, chassis and aerodynamic characteristics, engine fuel demands, and available torque. I'm not discussing in-town. That's far more heavily dependent on driving style, idling time (i.e. traffic), and as such usually not very comparable in my mind.
The thing is if you compare a 4000lb car to a 2900lb car with the same everything those differences marked are rare. Same when comparing a 2900lb Jeep to a 4000lb Jeep- often the economy's just about the same. Same motor will typically do a bit worse in a Jeep, a bit worse than that in a full frame Jeep, and than even worse in a full frame truck. That doesn't mean it'll have to do horrible, but it will do less. By the time you're typically talking of the utilitarian barn-door aero designed full size truck it's typically far lower, even if that full size truck is 200+lbs lighter than the car.
That I completely agree with. And I'll add I'm impressed you've seen 20mpg out of Wranglers. I think that's also why they've all pretty much gone to these VVT style setups so it'll cut power at cruise but it'll have the power under load to perk the maxes up even though they may be totally unobservable.
