[Bob Putnam]

"If you had (for instance) a 16 gallon tank, slanted on the bottom, at a gauge reading of 1/2, you would have 4 or 5 gallons remaining, not 8."

And that still doesn't take into account that The sender's resistance is proportional to the angle of the float, not its height. Of course you could just make the face markings on the gage wildly non-linear.