What is the rpm sweet spot for highway driving

If your bottom end is soft you will need more gear which will create high rpm on the highway and hurt mileage.Seems the best option would be a quality converter built your specs that will give you a better launch but still cruise nicely.
A lot of guys will not spend $600 or $700 on a converter but it is the best money you can spend.I would recommend Dynamic 9 1/2"