If you ever bothered to read your manual, you should fill up the tank when you reach 1/4 of a tank on the fuel gauge. this is for two reasons:
reason one, it's bad for the fuel pump, if you only have 1/4 tank or less of fuel, the pump pick up will suck air (especially when cornering) this can hurt the fuel pump, and your engine (sudden lean condition!)
Reason two is the tank full switch, as many of you may know, there was a recall on these, and it's what stops the gas pump when the tank is filled to capacity. bottom line is, if there is not at least 1/4 tank of fuel in the tank, the fuel gauge will eventually become inacurate.
on my GTS I have a 12 gallon capacity. when I get to about 1/4 tank, it takes about 10 gallons or so to fill it up again. So in reality, my tank may have a capacity closer to 14 gallons, but note that if you overfill beyond reccommended specs, your fuel system will suffer. bone dry, you shouldn't put more than 12 gallons in.
If you want to run with less fuel (less weight) get a smaller aftermarket fuel cell. a 6 gallon would be great, weighs a lot less than the factory tank too. just know that you will be filling up more often.