When we went to Bermuda in 2007, we didn't get much wind - so did a lot of motoring. I was really impressed with the economy of the boat - since I was monitoring the fuel consumption closely every day. After the third or fourth day of lots of motoring, the tank was still 3/4 full.
When the next day, the gauge showed only a quarter full, I realized the error of my ways - the triangular section tank and a linear fuel gauge! (and they called me an engineer!)
Fortunately, we managed to very sparingly use the last quarter (nay, fumes) and when we refueled in St George, we put 39.8 gallons into our 40 gallon tank!
While filling it, we also stopped every 5 gallons and I marked the gauge position in black marker on the tank itself. The next owner should be all set!
Formerly Toodle-oo! - and still looking for a replacement!