I keep very detailed fuel consumption records for my 85 300D, 177k miles.
At present, I am getting 23-24mpg and I drive a mix of 75% highway 25% city with the air-conditioning on. It rises to 24-25mpg or so if the a/c is off.
Best mileage I ever got was 26.1 this spring on a long trip. Worst was this winter when I got 21.5 a couple of times.