Rear diffusers come to mind. I bought some Carbon Fiber ones and installed them A real pain in the a*s. Then after it's all done, you can't realy even see them underneather the rear of the car unless you are nealing down looking at it from behind. They also tend to get scratched up underneath because the rear is now lower than before and I managed to scrape them when backing up in small parking lots. Should have replaced the rear bumper for complete new one instead of the add on diffusers.
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