Gas Mileage. Rumor from my airplane buds here in MI is the cross-member bars on the roof racks of the old Chrysler Mini-Vans and SUV's was costing them 2 MPG... Obama's mileage numbers and meeting them are everything right now..
Top speed is affected too. I read somewhere that the single bubble light on Audi police cars on the Autobahn reduced top speed by 20 mph or so.