I have just bought this car with 68xxx miles and a full tank of #91 gasoline could only run for 200 miles, so I really wanna know whether this is a higher fuel consumption and how to correct it

Another thing is that the rubbing noise always shows when I make a left turn, the dealer told me to replace the ball joint, I replace that the noise still shows but seems to be much smaller. There is a strange thing that after replace the ball joint there is a very small rubbing nosie which sometimes appears during normal driving (I mean just go straight).

I am new in American, I dont know whether I express myself clearly here, anyway, hlep me