I had an '83 Chevy van back in the late '80's that had the GM 6.2 diesel and 4 speed auto trans. It also had a camper interior in it, so it was hauling some weight. I could drive that van from Springfield Virginia to Lumberton, NC and back (can't remember the number of miles, but it was a long trip)  without buying diesel. At one time I figured out that it was getting 33 mpg on the highway, not bad considering it's size and weight. Loved the mileage that van got, until the injection pump failed....all those savings eaten up in one part!  Jeff