Its debateable whether a 2 bbl will save you any gas,because most have 2 fairly large barrels that are bigger than most 4 barrel carbs primaries are...
I've gotten better mpg from a 4 bbl ,a Q-jet or spread bore Holley will beat most 2 bbl carbs if you dont boot the secondaries in ...I liked the model 4360 Holley "Economaster" 4 bbl carbs,they would deliver up to 18 mpg on my 74 C10 that had a 454 in it...the Carter AFB's and Edelbrocks have very little difference in the primary and secondary barrels,but still can do pretty well on gas if jetted right...
I've used the smaller version of the 2GC Rochester carb used on 283's and 307's with good mpg,I could squeeze 16-18 mpg from my 81 G10 van with a 73 chevelle 307 with that carb,that has 1-1/4" barrels,but it has 2.73 axle gears so that probably helps a lot......the larger ones (1-1/2" barrels) were used on 327's and 350's and would burn as much gas as a 4 barrel carb ,they were popular on dirt track cars here,most were resticted to using a 2 bbl,so those and the Holley 2 barrels like Fords used factory were a common carb on many race cars on local tracks...
I've noticed most smaller V8's actually ran better with 2 barrels ,after I bought a manifold and a 4 bbl to swap onto them...the smaller displacement engines dont like a big open hole when you boot all 4 barrels in,the air velocity suffers and the engine bogs, or lays an egg instead of operating at full efficiency...a 390 cfm 4 bbl Holley works pretty well on them,but anything larger is a waste of gas,it wont add any power really..
One guy I know tried putting a one barrel carb off a 250 six by using an old 1 barrel to 2 barrel adapter he found at a swap meet for 5 bucks...said it runs OK,but is sluggish as far as acceleration,and so far its not much better on gas than his original 2 bbl was,on a '67 283 in an Impala...he wants to find a smaller one barrel,but I warned him he may start hearing spark knock and do some damage by running it too lean....
There isn't much you can do to a V8 to get it over 15 mpg in a full sized vehicle...the only exception to this I've witnessed was a neighbors '76 Lincoln that had a 460 with a factory Autolite 4 bbl,it would deliver up to 20 mpg,but he knew how to DRIVE it too....with that big an engine you only need to open the throttle a crack past idle...seen some 500 caddy engines do very well on gas if driven like grandma too...
I think the GAS they sell us is responsible for the poor mpg today,alcohol has less btu's and you need to burn more to get the same amount of power compared to straight gasoline...plus older engines were not designed to run on fuels suited better for todays fuel injected engines...
