While you don't get a Jeep for it's fabulous fuel economy, you should at least squeeze as much as you can out of it by making sure it's running as well as it can. When I was having problems with my carter my economy was low (~13mpg). The same for my Howell TBI setup - I just recently solved the EGR intake leak (needed to tighten the EGR tube nut)and with the last tank I got ~15 mpg, whereas before it was around 12-13mpg. Still ran decently though with that intake leak. While 12mpg seems to be common for YJ's, it might not be for your case.
Unless you are hauling some extra weight, tools, gear, accessories, a properly running carter should get you around 15mpg in the city, assuming no major hills to climb. I don't know what witchcraft plym49 has done to his carter
, the best I ever saw with the stock Carter was 18 mpg on the freeway (31" tires, 3.55 gears). A rack would create some drag though. Might as well do some checking to make sure that 12-13 is the best you can get out of your YJ.
I'm assuming you still have the OEM setup with the Carter -
-Start by looking for any vacuum or intake leaks. The computer will compensate for the extra O2 and dump more fuel to balance out the mix.
-Make sure the stepper motor is working properly since this is what affects the fuel delivery. It should be centered and constantly moving forward/back once the computer takes over.
-Make sure the carb isn't dripping fuel - either from the bowl or venturis.
-I'd also do a full tune up if it has been a while - plugs, wires, rotor, cap (Team Rush if not already done), fuel filter, oil change, timing. O2 sensor also.
An all electric engine in a YJ - the brick-like aerodynamics of a Wrangler probably wouldn't result in good mileage, also considering the weight of all the batteries. If you notice production electrics and hybrids, they are almost as much about aerodynamics as they are efficient engines. Put the 4.2L in a more aerodynamic shell and you'll likely get better than 20+mpg city/highway (seeing as plym49 already gets nearly that in his YJ)