When the S550 Mustang design was revealed to the public over two years ago there were admittedly some gripes about certain aspects of the car’s appearance. One of the areas that was in need of some major attention was the rear lower valance/diffuser area on the bottom of the rear bumper. On base model Mustangs this area is a single color, but on premium models it’s a somewhat odd split of two different colors, one being the body color and the other black plastic. The body color panel seems to float between the black plastic area, and for many Mustang fans this design just doesn’t work.

Photo credit: AmericanMuscle.com via Instagram account @5liter_shark

One of the first designers out of the gate with ways to improve the S550 platform was Vaughn Gittin Jr and his RTR brand. AmericanMuscle is the exclusive distributor for the RTR line of Mustang products. If you think the rear of your premium Mustang GT or EcoBoost needs a little cleanup, and maybe a more aggressive appearance, the RTR diffuser PN 387381, might be the help you’ve been looking for.

Constructed from injected molded plastic with a textured finish, this rear diffuser takes the place of the OEM part. It’s also compatible with the OEM backup sensors. This is a direct fit replacement, it does require a little wrenching, and it is a permanent replacement because you’ll have to do a slight amount of cutting to get the OEM diffuser out. Don’t let that scare you though, we’ve seen feedback from numerous AmericanMuscle influencers and customers who have said that the diffuser can be installed in under 90 minutes by most enthusiasts, the cutting is minimal, and the payoff is huge. Remember though, this part only fits premium package bumpers, so you won’t be able to install it on base model or V6 cars.

Photo credit: AmericanMuscle.com via Instagram account @projectwhitemare

The transformation to the back side of the car is remarkable. This is especially true for cars that have been lowered or are running aftermarket axle-back exhaust with tips that help to fill out the rear bumper. This is an attitude changer, the look is subtle enough to make the casual enthusiast scratch their head wondering how you did it, and impressive enough to be either the first or last appearance modification you perform on your S550.