3.55's are going to keep the motor a couple 100 rpm away from its Happy place....If it were me I'd skip 3.91's and go to maybe a 4.10 or even a 4.30 combined with a slightly taller tire...that way you're netting both a deeper gear and more bite at the same time.

With AUTOMATICS I've had and seen too many motors that really didn't see a huge difference (in et OR mpg) between a 3.55 and a 3.91)....by jumping more than 10% overall you can fine tune with tire size.