Diff change

[PAULJ]

I am running a 1275 itel engine with a ford 5 speed box. It runs and pulls well,my wife uses it daily. I would like to change the diff to reduce revs and get better MPG. At the moment I have a 4.2 diff and am thinking about a 3.9 or even 3.7. My question is will a 3.7 diff make it too slow on pull away-hills.
I would like to hear from anyone who has a 1275 with 5 speed type 9 with 3.9 or 3.7 diff. Thanks