Do you want it to read full when there's a short or when a wire breaks? I'd choose the least likely failure mode which probably is a sensor short. Then again, I would design the thing so that it didn't hit zero or max voltage with either a full or empty tank, meaning a wire break or short would send the needle off a scale in a most suspicious way.
Out of curiosity, what kind of sensor is it? Float? Capacitive?
Cheers,
/ft