Ah, you are talking about an entirely different wind here :wink: , usually referred to as 'slipstream'. A wind sensor would be not the best idea for this since the wind is 'apparent wind' that changes with vehicle speed and wether the vehicle is driving in a head or tailwind.
You could, theoretically, experience no wind at all while doing 60mph if you are happen to do that in an equivalent tail wind.
You might be better off picking up the trigger signal from some speed sensing device, perhaps you could fit an optical sensor to the final drive and use the output of this to activate your flap drive.
Now, if I get that right, the idea of a spoiler is to create downward pressure at speed. The flap would be there to fine tune that pressure. There might be a condiderable load on the flap so whatever drives it has to be able to stand up to that. Perhaps a worm gear drive? Good input to output power ratio, low output speed so you could use a small DC motor to drive it and, best of all, it stays put where it is if the power comes off. So, you gradually drive the flap up as the speed sensor increases its signal and drive it back to neutral as the speed drops.
You can find worm drives in hobbyist shops, perhaps they have a suitable one.
Its a bit more complicated than you wanted but I cannot see a solenoid do that job as reliable without being quite bulky. Also, solenoids are either on or off, no in between as the above drive allows.