[rockman20]

One that only goes to 3/4 when full, would leave me to believe it is a bad sender. The senders that I know (things may of changed on the newer stuff) went off from resistance. Kind of like how a light dimmer switch works. It has a row of coils and then a slide bar on it. Depending on where that slide sits on the coils, will determine how much resistance is there and the gauge will move to a certain point. If something has gone wrong with that set of coils or if the slide bar goes up so far and then gets stuck for some reason, it would only read 3/4 full.

Now this is assuming that the gauge still works once the tank gets below 3/4 full. As in the gauge still goes to the half, quarter, and empty marks.