I agree with Tech, but other things enter into it.

I swapped in a Holley vac secondary 600 cfm smog carb - jetted lean at the factory - for a 750 cfm Carter jetted/rodded for the altitude and found the mileage to be the same.
On a 462" Buick fwiw.

Pretty sure what happened here was the lean jetted Holley required more of a throttle opening than did the bigger Carter and pulled a little more fuel.


On the other side - and not sweating the mpg - little brother runs a total of 2200 cfm on his 500 cid engine....