not always true. when swapping the exhaust to an aftermarket catback like Greddy or HKS, expect your MPG's to drop. you lose alot of low end torque and gain some mid. it's noticable when you're at a red light and about to go ... the power just isn't there anymore. going from 50 to 80 is a different story.

another big lie is that exhausts add alot of HP. not true, you'll be lucky to get another 4 or 5 hp to the ground.