Nv3500 vs ax15

Higher gears are definitely required with the ax15. I was running 3.23’s in my Duster and still only using 1st gear if I was taking off on a hill. I’ve since installed 2.76’s and now use 1st through 5th gears as intended.
Wow. What must the final drive ratio be in fifth? 1.8 to 1? Usefully at all under 85 mph?