This isn't well informed -- Mirrors are a substantial source of increased drag. Something like 5% of the air resistance of a vehicle comes from the outside mirrors -- over the life of a car, that's worth several hundred gallons of gas, and across the population of cars, several billions of dollars in savings to consumers. I fret about the longevity of the technical solutions (mirrors on cars from the 60s work as well today as they did when they were installed) but it could be a huge boon.