Aerodynamics become active as soon as you start moving!
Seriously, you don't need to get up to 50 mph. You can do your testing at 35 with no problem (assuming no winds). There won't be any noticeable difference between 35 and 50. You have to get going REEEALLY fast (think jet fast) in order to substantially alter aerodynamic flow over a body.
If you have any question about how important aero is even at low speeds, click the 'tools' link on the top and go to the
Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com. This will show you your % of rolling resistance vs aerodynamic drag. For most vehicles, even at 30 mph, 50% of your energy expended is going to overcome aero drag vs rolling resistance. So, even at very low speeds, its still very important.