Maybe. It depends. In my situation, I believe a bigger carburetor will win out with MPG. On this Ford 400, with the Street Demon 625, it runs out of primaries real quick and I get into the secondaries pretty quick and I don't have a lead foot. I tried this Edelbrock 750 and when I had it running "good" I could tell a WORLD of difference on the primaries. But, it was not meant to be, because I just could not make the Eddy work well. I think if I am able to get the Street Demon 750 I will see a difference in fuel economy, since with the bigger primaries, I won't be into the secondaries "as much". It's a theory at least. I know beyond a shadow of a doubt now that it does need the bigger carburetor. So mileage or not, that's what it will eventually get.
