Make sure that the specific mufflers and all hangers are designed and guaranteed to fit your car exactly with no bending or drilling. Bending and drilling rarely can be done well enough to get a good, straight and aligned, non-rattling fit.

Usually the so-called performance mufflers will add no noticeable horse power, but they are louder than stock.

If they are freer-flowing than the stock mufflers, they may provide some very, very small -- and often not noticeable -- improvement in gas mileage. They will not likely "pay for themselves" in saved gasoline.

In practice since they are louder than stock they make a car guy drive more aggressively and, therefore, use more gas!

But they are more fun... if there is no drone at any speed!

PK, IL