17 gallons of diesel = 121 lbs (267kg) of additional weight to haul around as well.
In the 80's, I had a friend that purchased a late 70's Cadillac Fleetwood sedan from a former over-the-road salesman. It was equipped with the horrible GM converted gas-to-diesel engine. This fellow apparently didn't like to stop either, so he had two tanks in the trunk, one on each side of the spare tire.
I think "in those days" the availability of diesel was limited compared to today - that was probably some of the justification for the extra tanks.
For those uninformed, large GM sedans of that era had the gas tank mounted under the floor of the trunk, between the frame rails. The filler neck was reached by hinging open the rear license plate. The spare tire mounted on the "shelf" where the trunk floor rose up to follow the frame rails over the rear differential. In a full size car like this Cadillac, that left about 1 foot of space on either side of the tire.
The add-on tanks attached to the floor of the trunk and were contoured to follow the shape of this shelf, filling in the vacant space to the left and right of the tire.
Each tank held 10 gallons, which when added to the 26 gallon capacity of the main tank, provided nearly 1,000 miles of range between fuel-ups.
The installation was really cool becaused it used otherwised unusable trunk space. Not that the Cadillac suffered from a shortage thereof. Anyone shorter than 6 foot tall would have to crawl into the trunk to unbolt and remove the spare in case of a flat - that's how deep the trunk was.
This is one of the great things about diesel that is not available for gasoline-powered cars without special provisions: no need for vapor recovery, so the tank caps are sealed tight. The extra tanks in the Cadillac were gravity fed into the main fuel tank, with no valves required. You filled the main tank, then put the cap on and filled the extra tanks. They fed the main automatically. The fuel gauge would remain on Full for about 400+ miles of normal driving.
Of course, if you were stupid or otherwise uninformed, opening the cap on the main tank before the fuel gauge had begun to drop from Full meant a flow of diesel fuel out onto the ground and onto your gucci's...