Take the active spoiler on a Tesla Model X for instance - it is 100% a toy for people who enjoy the Transformers aesthetic. I don't think there are published numbers for it, but the Porsche Panamera which has a substantially more agressive spoiler is reported to produce a whopping 7 kg downforce at 250 km/h (150 mph), decreasing substantially at lower speed.
In the ordinary cars that actually need spoilers - famous example is the Audi TT - it's in order to fix crappy airflow giving lift at high speed, that is caused by the design of the car being optimized for looks rather than aero.
And aerobrakes?? Anything that's not big enough to cover 4 highway lanes is completely ineffective at speeds below 100 km/h (60 mph). It would be substantiallt more useful to install boat anchors.
Check out this fancar that set a new course record at Goodwood this year: https://www.youtube.com/watch?v=xtvT2XYlcOY
For anyone unfamiliar, a fan car uses motor driven fans to actively create a low pressure zone under the car, while also reducing the low pressure drag bubble behind the car. This particular one is all electric and quite compact.
It's very fast, and yes footage of it looks weird, almost artificial, because its sustaining grip through corners that rivals F1 cars, with the torque of electric. Running full tilt the fan system creates about 2000 kg of downforce. But the neat thing is you can flip a switch and then the thing just becomes a Nissan Leaf with a really crazy body kit. So some future "track day" car based on this concept would be surprisingly practical.
I think the future of auto racing in a pure electric era is going to be surprisingly bright.
Anyhow, not really relevant to talking about making big trucks more efficient, but thought you'd find it interesting.
This car is really cool, I'm not going to argue against that for a single second. But your suggestion of a "practical track day car" is missing the fact that there are two things making this crazy fast: huge downforce and ridiculously low weight. It weighs less than half of what a Nissan Leaf does, has a single seat, no storage for anything, no aircon etc. etc.
If you tried to make it approach the practicality of a Nissan Leaf while still being performant, you would not just need more space and weight for the practicality, but you'd end up tripling the weight since you would also need a much bigger battery pack to keep the runtime at the current ~30 minutes on a track, bigger motors to keep the acceleration high when you're dragging that weight, much wider tyres to enable cornering at these speeds, etc. etc. Then you're no longer killing hypercars but rather maybe matching a Bugatti Veyron, and you might as well just go for standard aero so you don't have the 120 dB (!!) fan noise inside the cabin.
But you said "Nissan Leaf". A five-seater car that can fit a full baby stroller plus a bit of luggage in the trunk. That is fully incompatible with being a lightweight track car.
Active aerodynamics are under-utilized. We start to see them, but they could be used much more, as break or turn assist, etc.
Imagine a car with retractable wheels. Only have a couple (or single) "bike" (narrow) wheel(s) for propulsion at highway speed, rely on aerodynamic controls for the rest, and on lift if necessary. Deploy aerobrakes and lower both the body and wheels in case of an emergency braking.
Such a system would be much more complex, but it could probably be engineered to be as safe as current cars (with aditionnal self- checks, attention to failure modes, etc).
I am not sure how much energy could be saved, but it's probably substantial. Plus people would finally have their "flying" cars (make them jump over detected obstacles too!).
There probably are some low-hanging fruits too, like deployable wheel covers for people who do not want to sacrifice low-speed aesthetics.