RPM at 70MPH

I'm putting a 5.7 with a 545RFE in my Duster, and have 3.91 gears to run in the 8 3/4. Even though the new Challengers come with 3.90 gears, and get good gas mileage....I keep second guessing myself about the 3.91's. I keep thinking I'd rather have 3.55's! :banghead: Then I just remind myself that the Duster will be a thousand lbs. lighter than a new Challenger so I'll quit thinking about it for a while!