There's a chart in the glovebox handbook for the W116 (non-6.9 and non diesel models) that shows the fuel consumption of the three engines (280, 350, 450) for moderate driving over a normal speed range and they are all within 2mpg of each other.
As young man interested in buying one of these cars I rationalised that the 450 makes the most sense as it has a lot more power and torque than the 280 and only uses a little bit more fuel.
Road & Track mag in the 1970s found the 450 returned 13mpg, and the 280S returned 15mpg. A few years later when the 6.9 was released they found it also returned 13mpg, so thats why I drive one now... I get more or less the same economy as 450 drivers, but loads more power and torque on tap. By 1978 R&T has pretty much said if you want better economy in the W116, go for the 300SE - 24-25 mpg and quicker than a 280S.