Don't ever stop asking questions! The most stupid one would be the one never asked.
As you've asked it, I'm unsure that the specificity of it has been properly addressed.
Here's some points to ponder:
1) The mirrors are only a fraction of the frontal area of the car.
2) Their frontal area and interference drag is part of the car, although one might wonder if the vehicle's critical turbulent boundary layer would be achieved in advance of the mirrors ( if you treat the mirrors as 'satellites' which orbit in formation close to the host vehicle ).
3) The small size of the mirrors suggest that their own Cd will not become 'constant' until a higher velocity is achieved.
4) Which begs the question of whether or not the 'vehicle's' Cd would require the higher velocity to 'flatline ' its Cd.
5) Convention has it that, around 20-mph, the vehicles Cd is 'fixed.' Warts and all.
6) If we accept that, then whatever percent of the total drag we attribute them to, that percentage would remain a constant, until the vehicle reached transonic velocity, where the air began to compress, and shockwaves could begin to form on certain areas of the vehicle.
Is that cloudy enough for you?
