318/340 Question almost on MPG

[1968 Charger B5]

My 74 roadrunner actually has a 318 in it right now.   it has true dual exhaust with full length headers.  i have an edelbrock alum intake with an edelbrock 500 carb on it.  its a stock 9;1 motor and runs great with only like 15000 miles on it, i rebuilt it.     now in my room i got a 340 motor my dad rebuilt years ago, with roller rockers, good cam, 9.5.1 motor old torker intake and carb,        my 318 gets like 12-15 mpg.......i am willing to suffer a few mpg for power, what do you think my 340 would get? 

oh i have a 904 with a small stall, and a reverse manual connected to an 8 3/4 with 323

[grouseman]

Put the 500 cfm carb and the dual plane intake on the 340, and I'll bet you your MPG is about the same.  IF you keep your foot out of it. 

[Chryco Psycho]

tweaking should gett the 318 close to 20 MPG

[whitehatspecial]

My 340 gets around 10-12 as near as I can tell. The throttle spends a lot of time in the open position though..