You can't expand your fuel tank size. Your fuel tank was
designed by the manufacturer with all the safety specifications and
calculations in place. It is located in the most optimal space in
your car there is, to prevent leaks in the event of an accident.
It's not so easy to modify your original fuel tank (as there might
not be enough space) and it will be highly dangerous to add on a
piggyback tank (for example, mounted in the boot.)
A better alternative to increasing your fuel carriage capacity
would be the installation of a CNG tank, properly installed by a
authorised installer. I hear that a full CNG tank can run about
200++km for some cars.
