When I drove it alot, I was getting anywhere between 23-27 mpg with my '83 300D. 95%highway..speed range between 70-80mph. Brand of fuel used most was Hess...occasionally Texaco ( with a "dose" of Diesel Fuel Catalyst).
Oil used was Castrol 20W50. A/C was on most of the time, Tire pressure checked regularly, and the tires where aligned. (the reason why I mention those things is because they can affect mpg in some fashion)
At this time it is getting a rebuilt engine installed. Once I get to drive it around a bit, I will check what this "new" engine will give me mileage wise.
-Larry

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D