Well long experience on this model by many owners basically means forty miles per gallon is not in the cards unfortunatly.

You can check the fuel pressure at the base of the injection pump. Or better yet read the thread on it under relief valve pressure. There may be a mile or two there per gallon. You should do this anyways as there may be long term damages running sub standard low pressure. Especially on the 616 engine.

I do agree that a modern diesel does much better. Part of this is the want and need for efficiency was not needed as much back when these cars where newer. Fuel was also fairly cheap.

I personally have looked many places on this engine design and only can hope you will find some way that has been missed to increase fuel milage. One of the very elusive five speed transmissions makes it a better car for general usage in my opinion.