The only spoiler I've ever had do anything at street speeds, is the one on my 72 Satellite Wagon:
(this isn't my car, I don't have a pic of it on this computer so I grabbed one online)
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Takes air coming over the roof, and pushes it down into the void behind the car. It actually CLEANS the back window if you're driving in rain or snow - and, by extension, I'm sure it also helps reduce the turbulence/low pressure area behind the car in the process.
Shame they don't do a similar design on cars today. My Grand Cherokee and my Wrangler could sure as hell benefit from something like this; their back windows are COVERED in crap when I have them out in the rain. And economically it would make more sense - one fixed spoiler, compared to wiring, switches, motors, mounts, and blades for a rear wiper.