I've been enjoying my-new-to-me 1991 350SD for a few weeks now. I'm slowly working thru the little issues but there's really just small stuff to fix. I like everything about this car. What a solid car this is! The only thing I would want is an overdrive 5th gear to drop rpms at highway speeds. It's not really a big issue but I think it could be a great upgrade.

I've found some old posts related to this but they went nowhere. So here's my question: would the 722.5 fit on a OM603? Has anyone done this and if so, how complicated is it?

The fact that there is little information available about this may mean it just doesn't fit or wouldn't hold up. Any input appreciated, thanks in advance!

It would probably fit, but the 722.3 is one of the more durable transmissions out there. You could probably change the final drive ratio to drop RPM at highway speed, not sure if the 350's were still 2.88:1 or if they'd changed to 2.65:1. Most of these diesels do just under 3000 RPM at 70mph, right in their torque band.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Thank you. I've been thinking about a different differential ratio but I'm sure it would negatively impact acceleration. I like how it moves now and don't want to give it up. As I said, it is not a really big issue, more like a nice-to-have if there is a straight forward swap. I'll do some more searching, perhaps I can find someone who's done it. Thanks again for your post!

Did any 722.5s make it to merica? 722.6 is far more common bolt on swap with several aftermarket controllers available. Not to mention the fact that it's far more durable.

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Thank you, I didn’t know about aftermarket controllers. Do you have additional information on those? Thanks in advance!

Home

https://mbturbo.com/new-722-6-gearbox-in-you-old-car/

Powertrain Control Solutions | PCS | Sound German Automotive

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Thank you, I will check this out!

The OD 722.5 was available in the R129 300SL/SL320 and W140 300SE/S320 through ‘96. IIRC it’s mechanical through the 3-5 shift but needs an electric signal to break that into 3-4 and 4-5 shifts.

Sixto
98 E320s sedan and wagon
02 C320

The 722.6 swap into my 300CE was such a great mod. Not cheap and quite a bit of work but easily worth every penny and all the time spent installing and tuning.

Another user here swapped one into a 126, though I don't recall seeing him post lately. You can search for his build in the performance paddock.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff

The 0.5 was also found in the 300 CE .

Before you go to all the work of a conversion, calculate what overall ratio you would have in 5th. Install some tall tires to mimic 5th and see if the car can pull top gear without constantly downshifting from original 4th to 3. Also, at high loads and low RPM, vibration might become an issue.

IIRC the 350SD has a 2.65 diff. I’ve read these cars don’t cruise as efficiently below 2500rpm because of turbo boost and TC stall profiles. For reference, E300s with 722.6 have a 3.46 diff ratio.

Sixto
98 E320s sedan and wagon
02 C320 wagon

Thank you, that's something to think about for sure! Not sure I can do the tire setup you suggest though.

Thank you. This may not be worth trying after all but I'll try to get more information on this. The 3,000rpm at around 72mph just 'feel' pretty high but if that is the rpm the motor operates best in then it obviously makes no sense spending lots of time and money to make it drop to something like 2,500rpm.

If you can cram W140 wheels onto your 350 you’d have the equivalent of going from your 2.65 diff to ~2.5. With the standard W126 tire size, a 722.6’s 0.83 5th drops you to an effective 2.2, or 2.0 with a 722.5’s 0.75 5th. Either is way too low unless you cruise at over 80mph.

Math class was a while ago so I hope these numbers are correct.

Sixto
98 E320s sedan and wagon
02 C320 wagon

What size tires / rear ratio do you have at present?

I'm not suggesting going to a larger wheel diameter ( metal part of wheel ) , I'm suggesting going to a larger overall diameter ( of the rubber part )

Lets put some actual numbers against this engineering project.