I had a 3.91 with 2-way Carbonetics LSD installed on my car this summer.
The shorter gear ratio compare to OEM 3.62 was a nice upgrade but the LSD was too agressive for me.
I recently installed a 4.10 pinion/ring in my OEM diff with the M-Variable LSD. I love it! It's so rev-happy compare to the 3.91. Even with my ESS VT-445, the 4.10 is a great match. I can't wait to try it next summer on the track.
I still don't understand why people choose a 3.91 over a 4.10???
Especially when NA