Gear change on 91 cummins rear end

[Cledus91]

My buddy asked me to swap out his rear end gear ratio on his 91 cummins from 4.10s to 3.54s. I know how to replace gears but I haven't done a ratio swap. I was wondering if any of y'all knew what's needed to change the ratio. I've heard that you would need shims when down sizing but I'm not a 100% sure if that's true any input would be great.

[Bushwacker379]

Yes the ring and pinion will be a different size. you have to shim the pinion and shim the carrier. You have to know what you are doing.

[Cledus91]

Ive changed truck rear end gears and car rear end gears for same size so I remember that part of checking the play and adding shims. I found some stuff on doing ratio changes as well as talking to my shops master tech. Thanks Bushwaker for the input.