Heh. If fuel is a weight factor, I have some thoughts-
1) Why can't we choose to fill the tank only as full as we need? For example, if a race is only 3 laps of Laguna Seca, 1/2 to 3/4 a tank from the start would be sufficient.
2) Why can't we cross-weight the cars in Gran Turismo? X-weighting is the art of making weights of opposite corners equal each other: LF+RR = RF+LR. More information is here ->
http://www.ground-control.com/?D=e04c39f1691068891bffa901ba6b5aaa
I've heard when you cross weight, you put the driver in the car and (for endurance races) fill the tank halfway. This is done so that for half of the race you are "over the curve", for a few laps you are "on the curve" and for the remaining portion of the race you are "under the curve." For sprint races, you fill the tank as full as you need to make it from start to finish.
It would be a major p.i.t.a. though if Polyphony & Sony made it so you had to "make weight" after the race in order to get your points. Having said that, it would be realistic and I would welcome it.
Making weight is more prevalent in a spec series: Spec Miata, Vette-Viper Challenge, American Iron, GTI Cup, Pro-7, and Honda Challenge at the amateur level. Speed World Challenge, ALMS, Trans-Am, NASCAR, and others at the pro level.
3) Also related: In theory, we should be able to use the weight of fuel to setup our cars in that "Weight Balance" setting.
Gran Turismo is a great racing game. It's not "The Real Driving Simulator." It has come a long way, but it has a long way to go.