Yeah, that is probably the optimal conditions - directly to and from work (so a good deal of highway driving and no real stop and go) and zero extra idling time.
Like I said, it's more like 16 lately due to the extra idling/warm-up time and winter gas quality.

I'm also driving like a grandma and have the manual tranny and corresponding lower rear axle ratio.

Tires are 235/75R15.

It probably doesn't hurt that I've done a new exhaust (manifold back), new NTK O2 sensor and tune-up with Mopar tune-up set within the last year.

In my '00 XJ, which is stock except for a 4 hole injector upgrade, I can get as much as 22 MPG on the highway. I do have factory rear mud flaps, factory running boards, and the roof rack on her. Which I am going to assume cause some drag. Also I added all three factory skid plates, full size spare, came with a space saver one, factory front and rear tow hooks, and swapped out the manual one piece buckets for removable headrest ones that are power on both sides. My point being is that I added a little weight to her. Now this is with using the cruise control, which I also added, and taking it easy on the skinny pedal. Around town is a different story. My mileage sucks. I blame that on the short trips. I live a 1/2 mile from work but drive this route so that when I pull into the parking lot at work the thermostat is just opening up and the temp is starting to drop. Betting if I drove farther to work the mileage would average out better. Also I have newer plugs in her, newer O2 sensors, and a newer exhaust from the front dual cat back. Also running some Cooper Discovery HT's. 225 75 15's that are in excellent shape. Mine is a CA Emissions XJ. It is what it is and I am OK with that. I have done what I can to get what miles per gallon I can out of her. Forgot to add that I did swap my front and rear diffs to a synthetic gear oil too.

As far as rebuilding and maintenance, I found my dad's Comanche was a little bit simpler in the electrical department. Not a heck of a lot of wiring and SUPER SIMPLE electronics layout. The 4.0 block also seemed beefier than the 4.0 in my Cherokee. With a total vehicle rebuild ( body panels, suspension, electronics, everything ), we managed to squeeze 220hp out of it. We modified the bed to accommodate a box large enough to hold and hide a simple doughnut from a 1997 Dodge Truck. Looks pretty nifty too lol. We made a furnished toolbox between the seats against the back window ( quite genius actually ) to hold the tire hardware, with spacing for a gallon of oil on one side, a wrench/ screw driver holder, and a gallon of Antifreeze. The tool box extends down into a center console where the 4WD and shifter are located. With some of this extra weight, his Comanche gets around 17mpg highway, 10 mpg city depending on his mood lol. Hes a electro-mechanical engineer and a machinist so his job is stressful. He put new Cast Iron manifolds, and the most basic muffler/ cat converter as it was born with ( despite my better judgement, it was his project ). He runs the basic 15'' Goodyear Wranglers ( He swears by em, as do I )
The Wagoneer; it was super simple, just expensive. VERY expensive. Its his "Family Toy". It gets a very efficient 13mph highway, 7 city. He rarely drives it lol. It too is a total restoration, with the exception of the engine... Yea, a 440 Commando ( SO EXPENSIVE TO BUILD OMG! ) but the interior and suspension are as original as can be.
My Cherokee, with a new head, Stainless Steel Banks Header, Magnaflow Hi-flo cat, Thrush Turbo muffler, new fuel and vacuum system (per factory spec ), original 1 hole Injectors still, and the basic steelies with Goodyear Wrangler 15s, I get about 23mpg highway, 14 city. Not horrible at all. My Cherokee has put up a fight, and its electrical system is of typical mid 90's Chrysler shenanigans, but it got manageable. Kept the replaced the original spare with a new one. It had a hole in it. I have made some basic improvements to the Cherokee, but I have no intention of nullifying them economy and performance benefits with any lifts/ larger tires lol.

I woant to get my wrenches on a 1993-1997 Grand Cherokee next, but economics, and lack of decent ones in my area of Ohio are a little difficult to find.