The 300SDL has a 26 gal. tank, just like the gassers, and can go as far 900 miles on a tank -- my brother ran to Louisville and back in his on a quarter tank once. Scary.
The 87 300D goes 450 miles, more or less, before I fill it up -- quarter tank left. If I could drive slower on the highway, I'd get about 32-33 mpg, but it drops off to 30 or so above 70.
On the other hand, the 300TE squeezed out about 22 on the trip to Cleveland, didn't write down the milage on the fillup there and the trip odo only goes to 0.5 miles and stops.
Peter