I added the diffuser first to the back of my car and didn't notice any smoothness change. The car did get quieter because of all the road noise in the hatchback i drive. The other aero threads state that sealing up the underside of the engine (front of car) is more important than the rear. Most rear bumpers are total parachutes so they do contribute to hurting aerodynamics, but you'll see more results with the front attack. Start with cardboard or coroplast as a stepping stone/template, to map out underside mounting points of your more high dollar materials. My 3 color aspire only deserves coroplast
