how much difference between 3.91 and 4.10

There are a few calculators on line to help you figure RPMS.
I used the one on SMR transmissions overdrive info page.
According to the calculator a 325/50/15 tire (BFGoodriches are 27.9 tall), running 60 mph would give you 2825RPM with 3.91 and 2960RPM with 4.10.