I Want to Build a Go Fast RC !

[SunnyHouTX]

Rear diffusers in an RC car are only mildly effective IMHO.

The way that Nic has his car set up in the photo that PBO posted, and the way we set up our RC drag car bodies, makes the whole body act the same way as a diffuser, but at higher effectiveness, and the down force is distributed over much of the length of the car rather than just behind the rear wheels (which will cause the front end to pick up).