Now I know where empty is!

Picked up a 76 D100 in the early 90''s for 200 bucks. Fuel gauge had 2 readings - 3/4 when it was full, and about 30 miles after filling up, it would only read 1/4. No more, no less. I'd go 225-250 miles between fill ups. If I was taking a road trip, I carried a full 6 gallon boat can in the bed, just in case.