For the best performance/fuel economy the manual states:
from 1st - 2nd: 15mph
from 2nd - 3rd: 25mph
from 3rd - 4th: 40mph
from 4th - 5th: 45mph
It's true does kinda balance the two out, but who wants to go the speed limit?
Plus remember higher octane does not mean cleaner burning or better fuel economy....