built up 318 oddly low MPG's? hp rating?

turns out I have vacuum secondaries. on a holley 650

I have 225/60/15 tires in the rear. I have another low profile single plane intake, I think it is a Holley street dominator. I thought the dual plane high rise air gap was good, how does that hurt my fuel economy that much? I just thought it made it flow better and atomize the fuel better. 318 heads