Changing gear ratio (archive)

I have a 1984 Euro 635csi. The gear ratio is 3.07.(for the autobahn, I guess). My mechanic has suggested a 3.91:1 diff. Any comments on whether this is too aggressive? Any downside to such a large change? (I want the quicker top end speed, but I don't want to give up much in terms of drivability.) Would a switch to 3.73:1 be better. Also, what is the best source for a replacement diff? Thanks. Rob.

Follow Ups:

Re: Changing gear ratio Bob Berry 12:41 5/20/2001 (4)
- Re: Changing gear ratio Rob Welch 12:50 5/20/2001 (3)
  - I would say that a 3.73 is the best, one ? Rob Anderson 14:59 5/20/2001 (2)
    - Re: I would say that a 3.73 is the best, one ? Bob Berry 20:46 5/20/2001 (1)
      - Isn't that what the car came with? I thought we Rob Anderson 23:49 5/20/2001 (0)

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