I recently took my 1980 300SD on a 3600 mile trip and it ran perfect. However, it got only 23 MPG. It seems to have good power, used about a liter of oil (castrol Syntec), starts instantly every time and doesn't smoke.
I adjusted the valves and checked the cam timing (3 degrees late) before I left.
What can I do to increase the mileage to the 28 MPG it should get?