closed chamber 714 casting 318 heads

On the CR calculator I use, I see 8.8:1...and that's with an .040"x4.15" gasket...typical for an OTS gasket set, and 0cc valve reliefs, stock bore/stroke 318.

Gas mileage is more dependent on your tuning, the type of vehicle, and how much lead is in your right foot.

Any benefit you feel will likely be on the off idle from fuel/air charge velocity.