My former 450SL, a 1975 model with the D-Jetronic did lousy around the city, around 7-9 mpg, but on the highway I could get as high as 16-18 cruising at 75 mph. In '76 came the change to CIS/K-Jetronic, so not sure if that was worse off for gas mileage?

Check for a leaking cold start valve, that drains fuel. Symptoms of that would be a rough idle. Fuel pressure calibrated, and fuel distributor adjusted for lean/rich mixture?

The 500SL (107) on the other hand has gobs more power and Im still able to achieve 22mpg consistently on the highway.