These V8 s do use a lot of fuel, they got 8 cylinders to be fed with fuel all the time, not 6 or 4. Gas is not as dense as petrol so will use more, mine i can only get 22mpg on gas is keep tp 55-60mph, soon as travelling at 70mph its down to 17mpg and above 70mph it only goes down....
Injectors can vary alot and cause less mpg, trouble is they are not cheap, even the not so good ones... just have to drive very steady....