+1 that,
when my procharger hits 12 or 13 psi @ about 5k rpm i bet i only get 4 or 5 mpg
also when ur crusin on the highways ur not boosting ur vaccuming which by the same mathmatics less air = less fuel but u still have parasitic load.
in anycase you shouldent sc or turbo ur car for gas milage.

 

It can increase gas mileage due to improved efficiencey, but most of it has to do with your tune and how you drive. My gas mileage on the freeway while cruising has improved by about 2mpg (I've seen similar results from others with a centri at ~same boost levels). However if your stomping on it all the time around town you will be lucky to see 10mpg.