I use linear potentiometers to gather suspension movement data and optimize mechanical grip. Downforce calculation is a secondary use that I give to them, however I take those numbers with a pinch of salt...
To measure drag I do coasting runs but they also include friction (tyres, brakes, bearings, etc)
While doing this, I check weather with a device similar to the ones you can buy in any nautical store:
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio
"I have no idols. I admire work, dedication and competence." -Ayrton Senna