I replaced my factory muffler on my `99 F150 4x4 ext cab 5.4 V8 with a Flowmaster 40-series.
That and a K&N filter w/ a modified stock air box netted me a best of 18.7 mpg hwy and some city driving.
It sounded very nice; throaty and noticeable, but not too loud.
"Liberty without morality is simply slavery to self." Me
"I don't think that counts as either unnecessary or inhumane killing. And the bonus of bacon falling from the sky can't be overlooked." jtmac, on killing feral hogs with tannerite.