[lunarhighway]

lovely car, but a bit of a sad story aerodynamically, considdering the opel omega B it's related to, replaced a car with an 0.28Cd! the prev omega had cut off rear wheel arches (semi wheel skirts) and was also more square in the back. I always thought the round corners at the back where part of the cause for the degraded Cd

what i can highly recommend is making a front undertray and next intall front and rear wheel dams(dont overdo thems just look at curent production cars they all seem to have them by now), and perhaps boattails or trakes behind the rear tires (i just screwed a lenght of plastic L frame under the bumer angled slightly inwards... seems to help)

I have been toying with the idea of using some sort of rubber insulation strip (like what you'd use on doors and windows) just ahead of a round edge to create a sharp cutoff. so perhaps your car might benefit from that just ahead, of the taillights but perhaps also on the trunklid

wide tires often seem to be the case of an increase in Cd when data is compared for a same model so if you can go for something more narrow that should also have an impact, although grip might also be degraded so that's a compromise

is the car a turbo? otherwise a warm air intake could work, at the expense of some power, however i have found that by leaving the original air intake in tact and adding an aditional warm air pickup ,(or forcing open an existing on in my case, that's only used during warmup) and adjusting the size of the WAI you can tune the air temperature to where you strike a good balance between drivability and economy.

interestingly engine temps seem slightly lower since i used a WAI (despite this being one of the hottest summers ever over here) so perhaps a little more grillblock could be an interesting side effect but YRMV