My W126 was not a 500 or 560, but the 420SEL. For comparison, though, consider that in Denver (higher altitude), I got 19 mpg on a regular basis, driving about 1100 miles a month, with a good deal of city mixed in. I suspect it ran leaner, plus I drove more highway (approx. 70 mph) than I do now.

When I came back down to The Swamp, the mileage dropped to 17.5. More city driving, 850 miles a month, with more stop lights.

I'd say you're doing great.
.

__________________
* *
-- Paul W. (The Benzadmiral)
('03 Buick Park Avenue, charcoal/cream)
Formerly:
'97 C230, smoke silver/parchment; '86 420SEL, anthracite/light grey; '84 280CE (W123), dark blue/palomino