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1998 SL500 - 1995 E420 - 1991 300SE - 1999 ML320 - 1972 280SE 4.5
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Discussion Starter #1 (Edited)
Hi all. I have a 1991 300SE that I use for my daily highway commute. Except for two stop signs, I drive exclusively highway miles at 75 mph+. The 300SE 3.46 differential is notorious for producing high revs on the highway as the price to pay for decent in-town acceleration.

Despite reading the posts where owners have contemplated doing the diff swap and have been dissuaded, I am pretty sure I want to go ahead and do the swap. My car has less than 90K, is cherry, and I plan to keep her a very long time, so I figure getting the right diff ratio will be worth it over the long run. And yes, I know my speedo will be off, but I think I can get a converter since my car has an electronic speedo.

I've searched the board but can't find the answer to my question. Please forgive me if there is a post out there on point that I overlooked. I want to do an easy, bolt-on, swap from the 3.46 diff in there now, to a 3.07 or possibly a 2.87. All non-LSD, non-ASR/ASD. From what I can tell from sorta on-topic posts, I can probably use a 185mm W124 diff from a 260E, 300E, or 300TE of the same vintage as my 300SE. I don't think I can use a W126 diff because the other ratios were with V8's and 210mm housings that won't fit my car's driveshaft and axleshafts. I'd like to just go out and find one at a junkyard and then DIY install.

Does anybody know what cars I should be looking for to be donors of a 3.07 or 2.87 185mm differential? particularly if I want everything to bolt right up easily without modification?

Thanks.
 

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'73 450SL, '83 300CD, '01 E320 4matic
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2,446 Posts
I often wondered why they didn't go with a different diff and just make them all 1st gear start. It would have been a little snappier in town and a little more fuel efficient on the highway.
 

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1998 SL500 - 1995 E420 - 1991 300SE - 1999 ML320 - 1972 280SE 4.5
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Discussion Starter #4 (Edited)
Aaron, I have wondered the exact same thing. The current setup would be good for in-town driving, that's just not what I do mostly.

I love my 300SE though. Ultra reliable and simple to work on for a DIY'er like me.

One other thing I love about my 300SE is the balanced handling. I've owned a couple of "L"s over the years, including the 560SEL in my avatar shot, but none of them handled like my 300SE. I don't know if it's the shorter wheelbase, the lighter weight up front and in the body, or if I just got a "good" one, but my 300SE is a real pleasure to drive. Balanced and sure footed. If I could just get it set up better for highway cruising...

Thanks lahiru for the suggestion on the 1987 300SDL differential. That will be like finding a needle in a haystack, especially one with low miles. Yet there are dozens of 300E/260E/300TE's right here in local boneyards. I'd really like to find out if these much more common diffs would fit my car.
 

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1989 300SE (Rear Ended & Junked) 1995 E420, Fully Loaded with Ortho Seats & Rear Sun Shade :)
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503 Posts
try a gen 1 diesel diff from a 300SD .. they're 3.07 .. somebody on the peachparts forum have done that change

Cheers
 

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1998 SL500 - 1995 E420 - 1991 300SE - 1999 ML320 - 1972 280SE 4.5
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Discussion Starter #6 (Edited)
I just spent some time on the other forum you mentioned. Looks like there are three differential ratios for W126's: 2.47, 2.88, and 3.46. The 2.88 would be the one to go with. Besides the 1987 300SDL, what other W126's used the 2.88? I'll start hunting one down. Or, if I could be sure that the W124 300E 3.07 would bolt up, I could find one of those very easily. I believe the 81-85 300SD would not work because it is gen1 and my 300SE is gen2.
 

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'73 450SL, '83 300CD, '01 E320 4matic
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One other thing I love about my 300SE is the balanced handling. I've owned a couple of "L"s over the years, including the 560SEL in my avatar shot, but none of them handled like my 300SE. I don't know if it's the shorter wheelbase, the lighter weight up front and in the body, or if I just got a "good" one, but my 300SE is a real pleasure to drive. Balanced and sure footed.
I agree. I had a 560SEL, and the 300SE is far more enjoyable to drive. The SEL always felt more like a large Cadillac than an MB to me.

With the SE, off the pedal you can understeer, on the pedal you can oversteer. It's an easy car to manipulate with the go pedal, and as a result it drives nicely through the country at high speeds. But you can also thank the diff ratio for that in a way- the 3000+RPMs at 60 mph gives you lots of on demand torque because you're in the power band. You'd lose that with the different diff. I'd be a worthwhile change if it helped economy though. On the other hand, I don't notice I get any better mileage at 55 than I do at 70, so it might not be worth the effort. Not to mention your speedometer will have to be corrected.

I think the gen 1 126's handle better than the gen 2's. I drove a beat up 300SD for awhile that had a much more glued down ride than either of the gen 2's that I've had.
 

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1989 300SE EURO
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271 Posts
And yes, I know my speedo will be off, but I think I can get a converter since my car has an electronic speedo.
Hi,

did you ever finished the swap? How did it go?

I am interested in that converter you mentioned.
How does this converter look like and where do you get it from?

Thanks,
Steh-fan
 

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1998 SL500 - 1995 E420 - 1991 300SE - 1999 ML320 - 1972 280SE 4.5
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22 Posts
Discussion Starter #9
No, I never did the swap. I could never get a definitive answer on what differentials would work. Lots of "probably'll work" but nothing for sure so I didn't want to take a chance.

I still own the 300SE, it's my backup car for when my E320 is in the shop ... like today, in fact. :rolleyes: Love that 300SE in town. Pulling up to the valet in the big black Benz gets attention from the valets who weren't even born yet when the W126 was made.

If anybody knows for sure which diff will fit, (the W124 2.88?), please chime in.
 

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I too would be interested in changing differential gear ratios in my 91 300SE....I mainly drive interstate speeds and the reduced rpm would be welcomed. Wondering if there is a way to reprogram the 4 speed transmission to start in 1st gear...haven't seen any discussion on that topic.
 

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1986/1990 W126
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13,362 Posts
Welcome. Someone a while ago posted about simply connecting a switch on some cars to effect a start in 1st gear. I don't remember te details, it will most likely be a case of searching the archives.
 

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Question - why would you want to reduce RpM when it puts you right in the correct powerband at highway speed ?

Is it just because we are conditioned to think low rpm cruising is “better “? For the six cylinder cars it’s not better to be at low rpm at higher speed
 

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560SEL 560SEC 300SEL E320 Cab. MB Metris Van / 300TD 300TE 300SDL, 300D, Unimog 406 (Gone)
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A 300SEL was my DD for many years and if driven 90% on open highway it would need a differential ratio change. At 80 which is about where I cruise the 560SEL on unobstructed road, the engine speed of the 300SEL is way beyond what is helpful, comfortable, efficient or economical. But, I got 18-21 mpg from the 300Sel compared with 12-14 mpg from the 560.
 

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W201 Moderator
89 190E2.6- 5-speed Manual, 95 E320 Sportsline-sold, 2001 E320 4matic Wagon-sold
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2,258 Posts
I do not have a 300SEL specific comment but even Mercedes kept tinkering with final ratio's on cars for the same W201, 190E2.6 model engines. For example, W201 with the M103 engine:

1984 - 1989 - 3.27 (I have this)
1990 - 1991 - 3.07 (probably a good compromise)
1992 - 1993 - 2.87 (perhaps good for 5-speed manual transmissions and predominantly highway driving)

I suspect Mercedes kept reducing the ratio to improve fuel economy and engine noise/rpm's at autobahn speeds.
Now that US highway driving is predominantly >65mph, I see a reduction in ratio is probably appropriate.

Unfortunately, finding a good differential that is not worn out in the wrecking yards with the desired ratio is not that easy these days. I've been patiently waiting.....

- Cheers!
 

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1990 SEC
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