Random thought. I know a full-on active aero and ride-height (like the McLaren 12C) is beyond the DIY capabilities of our community right now... but say we start with an active spoiler (like the Porsche 911).
Does anyone know of a successful (aftermarket) implementation of spoiler angle modulation based on vehicle speed? I'd like to think that we could either use OBD data or ABS speed sensor frequency to determine speed. A lookup table would provide the threshold values and outputs. A basic stepper motor control would manage spoiler attack angle.
If you could "pitch out" at low speeds and in eco-mode, then you could install a monster wing without hurting highway mpg. Similar control could be done on active grille shutters or front aero.
Just thinking out loud.
EDIT: Jackpot!
LINK to Supra LINK to Accord LINK to E36