I believe you can edit the xml to increase the maximum capacity without changing anything else. You will still have the same dead weight, and the additional fuel will still cost you. But combining this and a the mass scale in the tinker panel you should be able to make a more true to real life fuel tank in terms of capacity as a percentage of actual volume and dry weight vs full weight. However I've heard anecdotes about usage of mass scale mucking up the physics engine.
I believe you can edit the xml to increase the maximum capacity without changing anything else. You will still have the same dead weight, and the additional fuel will still cost you. But combining this and a the mass scale in the tinker panel you should be able to make a more true to real life fuel tank in terms of capacity as a percentage of actual volume and dry weight vs full weight. However I've heard anecdotes about usage of mass scale mucking up the physics engine.