When I fill my tank completely up my fuel gauge reads full. It seems to read consistently (as consistently as stock gauge can) until it reaches 3/4. At this point it drops dead. No fluctuation it just hits the bottom. Then it's back to watching the miles and at 200 I refill. What causes this?

 

The stock fuel level sending unit is basically just a float attached to a coil of wire.
As the float moves up and down with the fuel level, basically, a little wire moves across a coil in the fuel level sending unit. the wire on the float completes a circuit through the coil of wire. As a coil of wire has a set resistence, as the wire moves across the coil, the resistance will increase as more and more coils are passed over: this resistence is then interpreted by the fuel gauge to give the fuel level.
This works pretty well....

However, whenever you rub two pieces of metal together, you get friction wear. Basically, the little wire moving across the coil wears down on the coil. Eventually, the coil is worn right through and the wires of the coil break. When the fuel level sending unit is at the top, the coil is still completely... but as soon as the float drops low enough and the wire passes over the broken section of the coil, resistence goes infinite and the gauge goes to zero.

So, to sum it all: **** wore out. Happens all the time, more so on the gas-hungry TII's. You need to replace the fuel level sending unit in the tank. New part from Mazda is around $60, and it takes all of 10 minutes to replace. Might as well replace the $13 in-tank fuel filter while you are in there.

 

Thank you very much. That is the kind of answer I appreciate. As always the forum members help is much appreciated. Thanks again.