Best 300SD Rear Diff?

[amilio]

I am in the strange position of having a car that's spent its life in California, but was bought in and shipped from Washington DC. While the California models are usually lambasted for the added emissions equipment(none of which, to my knowledge was included in '79 SD's, but feel free to correct me), the California models came with a different gear ratio in the rear differential that made it more amicable to prolonged highway cruising.

My biggest peeve about the driving experience of the car has always been the annoyance of hitting 3000 rpm at 60 mph, and knowing how much an overdrive would fix the situation.

Wanted to get some opinions from you all about whether this is a possible upgrade to a rear diff (swapping in one off a 116/126 California 300sd), though I wonder if there is a modern drop in, or if this really doesn't do much at all.

Cheers.

[daantjie]

I'm pretty sure Kent at mercedessource had a video on this but might have been W123 related.

[daantjie]

Here is a clip where he talks about it albeit this is a 123:

https://m.youtube.com/watch?v=savjfjmGF-E&t=0s

[kjenkins]

   Thought I'd give my opinion on this matter. I've owned four w116 300sd's, one of which was a 1980 california model. They all had a 3.07 gear ratio.  There was not a difference between the california and federal models. If you find any w116 300sd that does not have a 3.07 rear end, its because somebody swapped in something different which can be done. A few years ago I remember seeing one for sale that had a 2.65 rear end out of a 1980 450 sel.  I believe the 6.9 also used this gear ratio. Somebody can correct me if I'm wrong.  This would allow the engine to be more relaxed at highway speeds, however it would also probable slow your acceleration from a standstill. Hope this helps.

[daantjie]

Yes the 6.9 has 2.65 limited slip diff.

[amilio]

Interesting to get some perspective from a CA SD driver on this. As Kent mentioned in his video, the diff would make things pretty slow off the line, so probably not a worthwhile trade off. While I could (eventually) swap the transmission for one with more gears or with overdrive, I severely doubt a modern one would last as long as stock.

Thanks for the input!