Answer:
That question is too broad. You must take into account other variables such as the size and weight of the vehicle, its tires, cargo, etc. You must account for wind resistance, aerodynamics, SPEED/VELOCITY, and any variables on the engine as not all V8 engines are the same.
My Jeep Wrangler has a V6, 35" off-road tires, and is lifted approximately 8" total. It only gets 13-16 MPG at ~70 MPH. While at the same time my Ford Explorer has a V6, 33" street tires, and is at stock height. It will get anywhere between 19-22 MPG at ~70 MPH.
You need to be more specific to find an EXACT answer (and even then it will only be a close estimate). Try listing the model of engine, size (or model) of the car, what type of gasoline, and the speed of the vehicle.