Yes, but...
In 1974 (?),
- nary a car on the road had overdrive
- aerodynamics were not commonly used... still not on XJ's...
- engines ran with "dumb" carburetors... not modern closed-loop fuel-injection
etc.
In short... I believe that in 1974, going slower MAY have saved fuel. Though, I still kinda doubt it. CERTAINLY every car has a "sweet spot"... based on aerodynamics, gearing, etc... and it's different with every car.
With that said... even within the realm of XJ's, it will depend on your axle gears, auto or manual tranny, tires, lift, etc.
My '97 Dodge Stratus did best between 65 and 70... because it had an overworked/under-appreciated 2.0L Neon engine and 5-speed. At 55, it was simply running too slow to be efficient, had to be downshifted for the slightest hill, etc.
Den
So there is no simple answer. Not even "55".