how many rpm's will i drop going from 3.91 to 3.55????????????????

I hated the 3.23 in my duster, with the 4 - speed I felt alot of chuga chuga on take off. My car came with a 3.91 so thats what went back in, to be honest with the small amount of highway driving I do, I don't see a big difference in fuel mileage ( at least thats what I tell myselfe) I would like to try a 3.55 or 3.73 though.