How much gas does a car use to carry its own gas?

Solution 1:

Assuming the fuel tank is half full on average, the density of fuel is $0.75 \space kg/l$ (ref.), so the mass of fuel is $25 \times 0.75 = 18.75 \space kg$

The car and fuel weighs $1218.75 \space kg$. The amount of energy used to transport the fuel alone is $18.75 \div 1218.75 = 1.5385\%$

The fuel used to carry fuel over the car's lifetime is $7.2 \times 3000 \times 0.015385 = 332.3 \space l$

Solution 2:

Gas is about $3/4$ the density of water, so a $50$ liter tank (on average half full) has a mass of about $18$ kg. If your dry car has a mass of $1200$ kg (easy to calculate with) and (as you ask) we assume fuel consumption is proportional to mass, you are using about $1.5\%$ of your fuel to move fuel. In fact, much of the fuel goes to overcome friction and air resistance, so the real loss will be lower.