Which Gear?

With that short tire if you want to do much hwy drivin you better go with the 3.55's or taller. According to my calculations with 3.55's at 70 mph you'll be turning about 3200 rpm which is pretty much screamin so you don't want to go lower than that. I personally would go with 3.23's if hwy rpm is a concern. 3.23 gears will still put you at 3,000 rpm at 70 mph.

If you want a close simulation of what 3.91 gears will be like go run your car at 70 mph in 2nd gear.