i want to get a better exhaust, but dont want less mpg‘s.
It depends on your setup, how you drive, and if you get a tune for your car. It will usually help your mpg, but not always. Cheers
Replace the muffler only. This may help a tad with mpg, nothing noticeable in either way.
Only if it's designed incorrectly. If it is designed correctly and the exhaust system can allow the exhaust to escape more efficiently, then it helps to increase your fuel mileage ( assuming your driving habits remain the same of course. )
