Well, let's see now...
A Carburetor works by flow. The more flow, the more fuel. Where you don't have RPM, you need to have manifold vacuum to create the difference in pressure across the carb and generate airflow to pull fuel from the jet ports. There is a separate set of ports for idle, and for off-idle operation. There is also a acceleration pump to add extra fuel when you transition from a lower RPM to a higher one to increase throttle response.
EFI works the same way, except that the computer calculates air flow based on MAP and RPM and delivers the amount of fuel based on a lookup table directly to the intake valve.
Between the two systems, there is no effective difference in operation:
More Flow = More Fuel
Less Flow = Less Fuel
Most 4 cycle engines with stock manifolds and stock cams generate the most torque on the lower end of the RPM range, and the most HP towards the top of it. The 4.0L has a fairly flat torque curve, but the HP is more slanted and the curves cross at some point.
Now this is where I start thinking out loud...
It seems to me that if you want fuel efficiency at Cruise, you find out what that point is, and set your gears for that RPM. Since the engine is most efficient at that point, you will not need to add as much fuel to achieve the power required for cruise.
The clearest power curve I could find was this one which was an attachment on JF.
So according to the graph, the engine will be most efficient at 2.6K or so, right?
The AW4 has a 1:1 3rd gear an a .75 OD gear ratio.
If you use this calc, you can determine the best gear to match your tire size, just remember to clear out the RPM line each time.
http://www.4lo.com/calc/gearratio.php
So as an example: 33" tire (We all know that's more like a 30.5") and a 4.56 gear with an Automatic (.75od) and the TC in 2hi (1:1).
Hit Calculate and we see we get 2264.
Running the numbers, we really need a 5.13 gear with 33's to get to the crossover point.
From experience is anyone getting 19 mpg with 33's and 5.13's?
Now this is the interesting part to me anyway...
From the factory, if all my thoughts on engine efficiency are true, then a Stock XJ with 3.55 gears, AW4, and 27" tires came with the wrong gears for the engine curve, infact, way off.
Where does that put the more fuel efficient Manual XJ with 3.07 Gears?
A AX-15 has a OD of .79, and though the calc we get 1962 rpm. That's way below the crossover point. Lugging down the road it appears.
It must only take about 50 HP to keep a Stock XJ rolling down the road @65. Somehow, that seems a bit low. Add a lift and huge tires, and I don't think that 50HP is reasonable at all.