I put an Am . Muscle axle back on my 2012 gt and my mileage dropped 5 mpg
That's a significant drop. It should have increased your performance and mileage (a little) if anything. Of course, this exhaust system has more to do with sound than it does performance.
Keep ur foot out of it
If the mufflers are more of a straight pipe like the SLP then that could be the cause for the drop
Is it creating drag
If you didn't adjust the Auto or Manual (Transmission / Flywheel) with an Automatic Tuner (plug-in computer pad) then your vehicles WILL significantly suffer. It's not allowing the new parts to be utilized , instead it's using Original Manufacturer's Specifications.
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