My 81 300D with unknown miles, it's on the 3rd speedometer, got 25.5 mpg a couple months ago. I changed air filters and mileage jumped to 26.5 mpg for a couple tanks. I plugged the EGR vacuum inlet hose because it seemed to smoke to much during a dead start and mileage has steadily increased to 28.4 mpg. Have you checked the EGR valve? There was no other work done on the engine besides what was mentioned above.