V6 Fi Vs 318

I have a holley 600 vac secondary, I had to do alot to it to make it run at the elevation i'm at. I would check your plugs and see if they are black or really dark, the carb may be running fat(rich). Have you done any work to the engine other than the intake and header?
Also what type of driving do you do a majority of the time, in town stop and go or mostly highway. With those gears your in town mialage will be down but should get better than 17 on the freeway. I run 4.10's around town and at the track and 3.23's when i take long trips on the freeway, Average 25 on the freway and 22.5 intown.
If you get into the engine mill the head to bump compression this will give you more power and increase your mialage as well, to an extent don't think a 12-1 comp lumpy cam race motor is going to get great mialage! Just get everything adjusted like valves and timing and get the carb just right and you should see some improvment in power and economy.

TF