Old Fella,
Your fuel guage may read in lbs, but the probes in the tank are measuring volume. They're usually temperature compensated to take care of thermodynamic expansion, but the basic SG of the fuel is just assumed to be standard.
I've done a few fuel cals where we put known mass quantities in an airplane and check them against the readout and you might be surprised to know how little accuracy there is. About +/- 5% is the best you can hope for. Remember, according to the FAA, your fuel guage only needs to be accurate when the ship is empty.