At least it's registering something. Let's assume your baby has ventured 100,000 miles over its
lifetime. That's about 7,000 tanks of fuel that she has gone thru. The TR3 fuel gauge itself is pretty sturdy assuming it hasn't been fooled with...but that Smiths BT3331/02 tank sending unit just can't handle that many rotations. I don't know what the factory specs were, but the six senders that I have on hand vary from 3 to 29 ohms empty and 81 to 96 ohms full. Full is no big deal, but that 26 ohm spread could leave you by the side of the road if you rely on your fuel gauge. David and Tom have the solution to make sure the float rotates free of the tank.
What I suggest (assuming your trip meter works) is figure out just how far you can get on a full tank of fuel.
Set your trip to zero, drive until you think your running on fumes, fill the tank, note how much you needed (14 gal. capacity +/-) and note the mileage. Reset your trip to zero each time you top off and use that mileage figure as you reference. You might also note where your fuel gauge was registering as a ball park reminder but the mileage method is better. To much trouble? Remove your sender and gauge, send it to someone who knows how to calibrate it and for $$$ you will have peace of mind !!!
Frank