Ok, I’ve used both, 3.70 and 3.90 Diffs in my 16 Hellcat. Not using stock ratio trans, 1-5 2.66, 1.90, 1.35, 1.00, .85. I switched from a 3.70 to a 3.90 when I went to mentioned trans ratios mainly to get the 3rd gear acceleration back I had with previous trans that was 2.98, 2.10, 1.43, 1.00...