On the highway, the 6 got gas mileage in the 30s at around 80 MPH which is pretty good, though below the EPA's 40 MPG number.

So you installed some aftermarket shit in your vehicle, and then want to trade it in to the stealership? Did you expect that it would increase the value of the vehicle by the amount you paid for it too?

At a price of $5500, he already lowballed himself.