I don't think your gas milage will suffer much. I went from 23 mpg ot 22 mpg after the supercharger. Going to 3.73 gears had a greater impact. Those dropped me from 22 to 20 (sometimes 21) mpg.
ProCharger P-1SC, 9 psi, STD 396/383; Uncorrected 388/375; SAE 383/370.
USAF 1969-1996: Vietnam veteran. Aircraft maintenance. ICBM Operations. Research & Development.