3.90 YES or NO?

3.90 would be a noticeable difference. I've got 4.10's in the Duster with comparable sized tires and it goes down the highway okay. I wouldn't want to drive it to Vegas, but it's fine for short runs and it really gets the power to the ground.
With 3.90's @ 60 mph you will be running just over 3000 rpms with those tires.
3.15's should be just under 2500 rpm's @ 60 mph now.