A search on the forum will be exhausting, as there's been many dozens of threads on this subject that I've caught myself up on the last few months while I've researched this myself. Regrettably, no single thread has
all the information. Also, the below just concerns the conversion of a manual car. For an automatic car, that is much more involved, but certainly not impossible. You'll need a different prop shaft, the manual flywheel, the pedal obviously, and probably some other stuff. That'll demand a bit of research on your part. There are some companies out there who do this kind of conversion, too, but I don't know their names or even if they're still in business -- just things I've seen from reading old threads.
There is a gearbox available from Siegfriend Thom over in Poland. I had it quoted delivered to me in Florida for about $7000. His kit should include everything you need. I believe it is adapted from a BMW 2-series gearbox, if I'm not mistaken. I didn't buy it because I don't actually want an overdrive gear.
You will regretfully not find any information about the conversion on this forum. However, Fantasy Junction has done it on a few of their cars. Here's an old article detailing exactly what they did:
https://www.fantasyjunction.com/uploads/documents/file/017990-how-many-speeds.pdf Note the cautionary bit at the end regarding lugging the engine. The overdrive gear, like that of the ZF, is great to use on the highway, but I wouldn't lug your engine just scooting around town. And by the way, here are the gear ratios:
1: 4.002
2: 2.108
3: 1.380
4: 1.000
5: 0.780
6: 0.645
Note that this is a double-overdrive box.
As for the five-speeds, there's a lot of information around here about using the Getrag 265 transmission from a BMW with a mechanical speedometer drive. These are getting harder/more expensive to find since the E9 owners have really consumed a lot of these, but they're still out there. If you look up "mkbull" or "Mark Bull" on this forum you should find some information about the kit he developed. It's unfortunately hard to find all of his posts since this forum did some sort of change to the accounts some time ago which made unpaid users show up as guests, so you can't just click on their name and find their posts. Anyway, doing the Getrag conversion is a bit more involved.
There's also the ZF 5-speed. A ludicrously expensive new unit was made available a few years ago. But I assume you're not willing to spend 20k for a finicky gearbox, or probably 6-8k for a good condition original one. I have learned from Benz Dr that the ZF 5-20 you see around
must be from a Mercedes, not from the Maserati Mistral or the other two cars it was available with, as the tail stock and bellhousing are different.
I'd also advise you that depending on your needs, you may find a 3.27 rear axle to be more than satisfactory if your goal is just to reduce overall RPM. My understanding is they are cut down by like 500 in the 4th gear with that axle if you started with a 3.92 or a 4.08. I believe all 280SLs came with a 3.92. Dropping to a 3.27 should be comparable to using the overdrive gear from the ZF if you keep a 3.92 axle. Shameless plug, I have one for sale with an uncommon limited slip differential, a very nice thing to have. I bought it thinking I was going to add significantly more power to my 230SL but then my goals changed with my car so it has been mothballed.
Should you go with the Getrag option, you have two choices. Getrag 265/5 and Getrag 265/6. The former has the 1:1 final drive ratio. The latter has the overdrive gear. Here's a comparison of the two proven overdrive 5-speed options. Sidenote: I have heard about a Tremec conversion, but I haven't found much information. I believe it is heavier and is a larger unit that may not clear the tunnel.
ZF S5-20 Getrag 265/6
1: 3.92 1: 3.82
2: 2.215 2: 2.20
3: 1.418 3: 1.40
4: 1.000 4: 1.000
5: 0.848 5: 0.810
As you can see, the Getrag is basically the same as the ZF, and it's the no-brainer decision if you want the overdrive gear, if you're starting from scratch. Also, FYI, I believe the stock Mercedes 4-speed post-1965 has these ratios:
1: 4.05
2: 2.23
3: 1.40
4: 1.0
Good luck!
