Stupid question time.

[Matt]

That's gonna vary a lot based on driving conditions and style. If MDS doesn't turn on often for you, you're not going to notice much of a change at all. But if it's on for 90% of your driving, you'll probably take a significant hit.

I'd say, get the programmer first since it's useful regardless of the exhaust. Disable MDS and drive around for a month or so to see how the gas mileage is really affected.

[pjmjr508]

CT I have mine turned off & running with a 3" Flowmaster with 50 Series Cat Back. It is a mean sound so you will have your big block hot rod sound & my mileage is around 10.5 to 11. I do 90% City, little freeway driving. I also have a kinda heavy right foot as well.

Hope that helps with your question.