Something tells me that the fuel gauge or a part of it isn't reading correctly. There's no way that one of these could achieve anything above mid 20's MPG on the highway.
Funny you should mention the good economy. I used to drive a Fifth Avenue, which is the same car, and anytime I ever went for a long drive, I was always getting around 35 on the highway, and that was at 70mph. If I only knew the RPM of the engine at that speed; it may have been as low as 1100, considering engine noise was never audible.
I once heard someone saying that the lean burn computer controlling the timing and fuel mixture nearly almost had them stalling at all times, no matter what RPM they were running at, and that the low down torque was why they managed to turn over at all.
My old 318 Plymouth was an incredible car and extremely reliable. My best mileage, however, never topped 20 MPG even on the highway. It got about the same mileage as my V-8 Mustang, which was about 10-11 in town and 15-17 on the highway.
True, a late 1960s to early 1970s era 318 with automatic can be expected to consistently get between 18-23 MPG on the highway. However, maybe the lean burn and lock-up torque converter of later years added some mileage. Still, 50 MPG from a 318 automatic, even with a lock-up torque converter, seems like a stretch. Even 35 MPG. I don't doubt for a moment that the people genuinely believe what they're saying based on what they know, but I would need to know that the odometer was working properly with correct tire size, and that the gas usage was determined by starting out with a full tank, then refilled after the drive to check consumption. I'd sort of need to see that for myself.
Two possible scenarios here:
The first is that some 30+ plus years ago, Chrysler figured out how to get twice the current CAFE requirement from an old-design 8 cylinder pushrod engine with a carburetor, but for reasons unknown decided not to place this engine into their production vehicles. Instead they invested millions of dollars in developing new technology engines and induction systems to meet the CAFE requirements. However, one of these "miracle V8s" accidentally got installed in the reviewer's Gran Fury.
The second is that the MPG claim is either a complete fabrication, the odometer in the reviewer's car is wildly inaccurate, or someone is utterly incapable of doing basic division.
I'm betting on the second.
Considering how damn durable and reliable the 318 was, maybe you could say it was a miracle V8.
The Gran Fury, 5th or Diplomat had a light body weight of 3700 pounds max, and the Torqueflite a904 transmission, even though only a 3 speed, certainly had one hell of a ratio in top, and coupled with the 8.25 rear, I bet on a cruise it would give respectable mileage considering the engine's 5.2 litres. But I say the best would be a possible mid to high twenties.