[aerohead]

Removing the Fusion's mirrors would drop the Cd from 0.34,to 0.325.A 4.41% reduction.
At 55-mph this would net you a 2.2% mpg increase and no outward vision.
Without a wind tunnel you'd never be able to see this 2.2% improvement through the signal-to-noise ratio of road testing.
These quanta would be specific to the Ford Fusion.
Every car would be on a case-specific basis.No generalities could be inferred.