Active Aerodynamics

I propose the installation of vortex generators on "boxy" vehicles to reduce drag in the trailing wind-shadow of the vehicle. Candidates would be vans, the 'Transit', Explorer, etc. Vortex generators would lay flat on the roof and/or rear quarterpanels until the vehicle reaches highway speed, then they would deploy. The fins could be manually deactivated if carrying lumber or a canoe on top, or similar situation. This modification would not be real expensive but could dramatically increase highway fuel mileage. Would also improve trailer towing. Fin size would be proportional to the vehicle.