It's a 10 gallon tank essentially right.. 8.7 main tank with 1.3 reserve. I'd put a couple of filled gas cans in the car and run it till you run out of gas.
I wouldn’t recommend riding around with gas cans in a car if I could avoid it. I don’t like doing it in a sedan with a trunk and certainly not in my ForTwo. Your car’s gas tank is designed and located with some measure of impact, vapor, and flame protection in mind. A loose gas can? Not so much.
If you’re concerned that the gas gauge isn’t registering properly, you should go refill the car’s tank and see how much you’ve actually used.
If it’s only a gallon or so then you might be worrying prematurely. My 2009 Passion doesn’t start registering until after 75 or so miles, but then becomes more accurate the rest of the way down.
After refilling, zero your trip meter and then watch the gas gauge over the next 250 miles. You should eventually see it move, and will learn your individual car’s behavior. If it still doesn’t change, THEN you may have a problem with the sensor that needs to be checked out.