I just bought a 1983 MB 240D and I only got a best of 28 MPG on a road trip. I was driving pretty fast and pushing speeds up to 85 MPH, but it still seems like a bad MPG for a 240D. Does it have something to do with the idle speed control being too low or too high? The car sounds really loud at that speed (80-85 MPH) and feels like it needs a higher gear, is it just geared low?

Also, the car does not want to shift down without manually moving the gearshift from "D" to "S", or from "S" to "L" at really low speeds. From a standing start, it does not feel like it's getting full acceleration and engaging in first gear. It does not slip, but shifts harshly. The downshift button beneath the gas pedal when the pedal is floored does not trigger a downshift either. Any advice?