Yeah I guess there are many factors (a given). However I recall reading an article quite a while ago on autospeed.com saying that one way to measure the effectiveness of your drag is to go on the same stretch of road, same weather, conditions etc etc and accelerate to ~140km/h and let go of the accelerator, then time how long it takes to drop back down to 100km/h. This only works for the same car, but my old Subaru (newer actually, 2004) had basically the same gear ratios and roughly same weight, added AWD, and still noticeably would drop down slower from 140km/h if i ever tested it. I know cause there is a point on the freeway where I always coast, that had larger and heavier wheels than stock, compared to stock 16" GTS-T rims and tyres.