The average size of a small to mid-sized car's fuel tank is 12 gallons. There is no standard fuel tank capacity, as automobile manufacturers choose fuel tank sizes on the basis of many factors, including the design and fuel economy of the vehicle.
The Toyota Prius, a compact hybrid car, has a fuel tank capacity of 11.9 gallons and can achieve an estimated 50 miles per gallon, as tested by the Environmental Protection Agency. The Chevy Volt, however, has a fuel tank capacity of just 9.3 gallons but achieves an EPA estimated 98 MPGe, which includes mileage achieved using the car's electric drive.
Larger cars can have fuel tank capacities of more than 16 gallons and trucks can have fuel tanks of 25 gallons or more and may even have more than one fuel tank.