Without being sure how the car is driven, your expectation shouldn't be for much more than 25 combined city- highway. To get it at peak performance, it's fairly simple- new air and fuel filters, frequent oil changes, proper valve adjustment and IP timing. Also get timing chain checked for excessive stretch. Take note of any other symptoms of poor performance- excess smoke, significant speed loss on steep hills (once cruising at 65, it shouldn't drop below 60 on a long steep grade). Also, make sure the brakes are'nt binding, and that tires are well inflated to at least 32 PSI- I keep mine at 36. Anyway I just did a pure highway trip with my '76 300D yesterday- 273 miles nonestop at 70 MPH, and got 29 MPG Imperial (Canadian) on winter Diesel, which translates to approx 23.5 U.S. With summer Diesel I was getting a little better but not much.

Dave

'76 White 300D W115 "Pearl"