Dont forget the transmission is geared for reverse with the 3.46 diff most likely. with the 2.88 or even the 3.07 the reverse will be too tall. I have the 346 in mine and i have not added the tach but it feels very nice. The NA 300d 4 speed w/346 will drive at 80 mph all day with no issues. With an overdrive it will be much nicer of a ride. Keep the 3.46 its designed for that transmission IMHO. Who really wants to go much over 80mph in a w123 anyway.

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Chris
84 280sl
82 300d euro