I always thought spoilers on cars were ineffective until around 70 mph.

however, they do say that it's more fuel efficient to roll your windows up and turn the AC on as low as 45 mph.

as posted, spoilers are no more than airplane wings. except they are upside down to produce downforce instead of upforce (lift)

lift causes drag, no way around that. the more lift a wing makes, the more drag it's going to have.

think big heavy cargo plane. the wings are so thick (I can almost stand up inside a B-52 wing at the wing root--I know because I've been inside the fuel tanks), that they produce massive lift at low speeds. they also produce lots of drag at low speeds. that's why cargo planes are slow.

think of an fighter airplane wing. very thin, and sleek. (I probably couldn't even fit inside an F-16 wing) they produce almsot no lift at slow speeds, and need to travel much faster to produce lift. but, it's their mission to fly much faster, which is why they have such sleek thin wings.

so, the shape and design of the spoiler will drastically affect the amount of downforce and drag it will make.

I think an AAR/TA rear spoiler will produce more downforce and drag than the sleeker go-wings that stick up off the trunk lid.

but, I don't know, and have no real world numbers to support my claim.

there are other ways to improve handling without adding drag inducing spoilers though. simply lowering the car will lower the center of gravity, allowing it to handle better, AND by lowering the car, it allows less air to get trapped under it at highway speeds, which means less air pushing up on the car from the bottom, so by simply lowering the car, you get less drag, and better handing.