Right figures! £29.40 is the cost of topping up. Capacity of the fuel tank? Yes, 28 l is the amount of fuel purchased for topping up, but the capacity of the tank is a gallon more than this. 28 l = 28/4.554 gallons = 6.16 gallons plus 1 gallon of fuel already in=7.16 gallons, the capacity of the tank.