I'm interested to know if it does achieve 19-20 mpg once broken in.
I just came back from a 2600-mile road trip. Most of that was fast highway driving (boy, do people drive fast), some - local errands in sub-freezing temps, so these mileage numbers include generous warm-up times. The trip included many, many mountain passes of ~3-4k ft above surrounding areas.
The return trip (after ~1400 mile mark) was with a foldable baby crib strapped to the roof bars - which you can imagine takes some gas to move at 90+ mph in Utah and SoCal.
True average mileage - total fuel / total miles - was 14.23 mpg. It would have been very close in an 5.0V8 LR4 - it had been in the past.
The best gas mileage was achieved, unsurprisingly, during a white-knuckle drive in Wyoming/Idaho/Utah during a snowstorm, at speeds
under the posted limits, sometimes significantly under.
The worst - on I-15 in Utah, where the consensus of the non-big-rig drivers is in the neighborhood of 95 mph with a posted limit of 80, followed by U.S.95 in Nevada with a stiff headwind.
Sidebar: you can't really trust the fuel gauge and "remaining miles" estimate. I came home today with "120 miles remaining" on the dashboard; unloaded the truck, took off to a carwash, and it said "fuel low, 44 miles remaining" - a bit annoying. Our LR4 was far more reliable in this department (maybe the only one).
I don't believe P300 and P400 Defenders get better gas mileage in comparable conditions.