Slick 60's

I have found out that the 1 barrell carb on my 223 is not the original, it seems that somebody took a carb from a 300-6 and put it on my 223, now my question for you carb experts, I'm using alot of gas, only 9.5 miles to the gallon to be precise, I'm sure that the jet for a 300-6 verses a 223-6 are different, is there any way to calculate which would be the right jet to use, for a 223.

What carb. is it? My 223 has a holley 2043 1 bl and get about 17 hwy /15 city with 3.70 rear gears.
Charlie

223-6, carter 1barrell off of a 300-6, with 3:89 gears sorry I didn't post that

Sounds like it may have a bad power valve. Good luck.

i found a bunch of carb manuals on oldcarmanual.com