1979 300D Non-Turbo Mileage

My 300 non-turbo with 360K at best gets only 19 miles to the gallon of diesel. What is this car expected to get when running perfectly? I have owned this car for 2-1/2 years now, and the mileage has always stayed in the 17 to 19 mpg range. Other than that, the car has the usual issues that the older cars do, like worn vent pods inside the dash, old rubber seals on the rear window and doors. Is there anything that I can do short of disrupting the performance to increase my mileage? Thanks.

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