[wootwoot]

^^^ he is in Europe and the car came with the engine in it...DICK!

exhaust leak would not cause poor gas mileage
a failing piston ring would not cause poor gas mileage, just burning oil and less power

Are your fuel lines clean looking or do they have an appearence of any resiude? The fuel lines wont look wet if they are leaking almost ever because whatever drips will evaporate extremly quickly. That seems unlikely though. How does the car drive currently? You never said if you noticed a good amount of black on your bumper/exhaust as well