i just replaced the factory muffler on my 99 4.6 and gas mileage went in the crapper. went from 16.5 down to 14.1 anyone else see drastic changes such as this. oh i put on an old school 40 series Flowmaster. i love the sound of it and maybe thats the problem with the economy, i cant keep my foot off the loud pedal now.

 

i think you anwser your own question, you are hitting the gas more and that is why MPG went down give it a week till you get used to it and it most likely will go back to normal, good luck

 

The lead in your right foot caused your decrease, not the muffler.

 

i think you are correct. hopefully soon i can get off the gas and quit enjoying the wonderful tone. thanks.

 

Ive got a F-150 reg cab, 8' bed 4x4 with a 5.4L and a Flowmaster 50 Series and I can pull 18 mpg on the highway @ 70 mph, so maybe it is your right foot

 

lol... oh if i had a penny!

 

Bigtruck is right, you did answer your own question.