Good experiment, especially given the budget...

If you had a barometer and a thermometer you'd have enough info to solve P=rho*R*T but over the course of your test series it seems unlikely to change much and not enough overall to change the conclusions for different weather.

If you are interested in a condition where drag doesn't matter - one of my longtime online aquaintances was a judge recently at the NASA rover challenge where high school teams build folding 4-wheel offroad HPV's and ride them around Redstone. I can't think of any way that streamlining would help those guys.