If you use good components, it takes about 4 hp to the compressor to get 1 hp out of an air motor. Home made compressors and motors will do far worse.
It's because of the laws of thermodynamics, specifically adiabatic vs isothermal compression and expansion.
The vacuum gauge plus wheel covers helped increase summer 2015 mileage to 38.5 MPG, while summer 2016 mileage was 38.6 MPG without the wheel covers. Drove 10,588.6 miles in 2016 at 35.0 MPG.