I've replaced both rear half shafts on my 240D. While cleaning off the differential I noticed that the rear ratio is 3.69:1.

I know the 300 series have a "lower" ratio.

Are these a bolt in replacement? I'm not necissarily looking for better fuel mileage (but if it was even marginally better that would be a bonus) but more of a wear factor.

I drive almost 100 miles a day, most of it highway, and lowering my engine/tranny RPM (even 10%) would be beneficial to me.

I have access to a very inexpensive 300TD diff and can get a 300D diff.

Any information would be appreciated...I used to have a link to which diff would bolt in, but never thought I'd change mine out, until now. I didn't realize the ease of replacement on these units.

The diffs are pretty much a bolt-in from the 123 chassis but keep the spacer rings that go between the half shafts and the diff body, they are shims of different thicknesses sometimes you need to use the ones that were on the donor car to prevent any gap on the new diff (causes a slap noise when you shift gears).
That brings up the important question, is your 240D a stick or automatic trans? If a stick you will be able to run along with only slight changes in shift points whereas if an automatic you may have to wind up a bit longer and therefore not gain as much potential fuel savings. I would only go one "step" in diff ratios with an automatic.
Watchout: the 300D can be a turbo or a non-turbo. A non-turbo 300D may not offer a diff with much difference, I think they came with a 3.46. I had a table somewhere that lists the diff ratios on the 123, I'm sure there is one on the Shopforum. Use the search feature to look for info.

You may be able to go as far as 3.07 which is the diff used on 300D turbos (called 300Dt by some) but I suggest you avoid using the 2.82 off an '85 300D/TD, that would probably prove too tall especially if you drive up steep hills a lot or carry a number of passengers or whatever. It simply won't match the torque curve of a 240D well and you may wind up lugging the engine (bad on bearings and pollutes).

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

Other way, they have a higher ratio.

They are a bolt-in replacement but you need to replace the speedometer part of the cluster with one from the same year/model for it to be accurate.

I wouldn't go more than a 346 diff in a 240d and probably not that unless it had an automatic tranny.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

LOL, that's why I put this in quotes...I was a "regular" mechanic for years and it didn't matter how this was stated someone would correct it!

I find it comical now.

I do have an automatic, and still run 14" tires (I know a lot of guys have "upgraded" to 15"). I purchased my MB from a former John Deere Mechanic (JD actually did a lot of the R/D on these motors and tranny's) and he set up the shift points on my transmission. I think a lot of the problems guys have with take offs is that the MB's sold in the states shift almost immediately from 1-2 basically making the transmission a three speed.

I understand the point about the engine bogging or not being in the power band at Hwy speeds...but I am caught in traffic that regularly cruises near 80...if I'm not at 70-75 I honestly feel like I'm causing a dangerous situation.

I can keep my "girl" at 75, but would love to lower my RPM by 10-15% while doing this. I know these engines were designed to work, but I think it would ALL last longer if she wasn't turning such high RPM.

Thanks guys!

Going to a 3.07 would be a 20.2% RPM reduction, 3900rpm to 3200rpm at 75mph.

Thanks for the info.

I'm thinking of also going with a 205/75R14 tire (a little over an inch more in circ.) which would lower me to around 3000 RPM with the 3.07.

I love this site! You guys are awesome!

Thought I'd chime in on this thread as I'm wondering about the scenario....I've done a manual trans conversion on my 84 300CD, and have really high RPM's at 65-70 MPH. I put the 240 tranny in and other than high rpm's at highway cruising speed, everything else is great. I was just wondering if pulling a 240 differential would be better suited with that tranny. Not to sure on the ratio's of these but wondering if there are any thoughts on that one. Cheers,
Jason

your 84CD should have a 3:07. scrape off the grease and dirt off the lower left corner of the diff. the ratio is stamped in the metal.

going to a 240 diff will give you higher RPM`s. If you want to drop the RPM`s at highway speeds, go to a 2:88 out of an 85 300D or 300SD.

Charlie

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there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

4th gear is 1:1 in both the autos and manuals.

2.88 is too high for a manual.

But you appear to condone putting a 307 into a 240d above....

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I have a 82 300CD turbo, 4spd manual and 2.47. I think it works just fine. It is a little slow off the line but it is awesome on the highway. I run 85mph at 3000 rpm every morning coming in to work and still get 28-32 mpg

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1982 300CD Turbo (Otis, "ups & downs") parts for sale
2003 TJ with Hemi (to go anywhere, quickly) sold
2001 Excursion Powerstroke (to go dependably)
1970 Mustang 428SCJ (to go fast)
1962 Corvette LS1 (to go in style)
2001 Schwinn Grape Krate 10spd (if all else fails)

A 3.07 is not a 2.88. A 3.07 is borderline in a 240D, less so with an automatic.

Its all about the point of diminishing returns. With a 2.88 the engine RPM may be lower but a 616 will consume more fuel working harder to push the car.

You also have an extra cylinder and a turbo to increase torque.

I had a 1982 240d that had a 617 na engine transplanted into it.
It was automatic, and i installed a 3:07 diff from a 1982 300d Turbo.
It was great on the highway, but a little slow off the line, especially with passengers, and would hardly go uphill cold..
With the 616 engine, I would recommend you only go up one step in ratios, get a 300d NA diff, should be 3:46

true, but I was answering the post of someone with an identical car, I agree this most likely wont work for a 240

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1982 300CD Turbo (Otis, "ups & downs") parts for sale
2003 TJ with Hemi (to go anywhere, quickly) sold
2001 Excursion Powerstroke (to go dependably)
1970 Mustang 428SCJ (to go fast)
1962 Corvette LS1 (to go in style)
2001 Schwinn Grape Krate 10spd (if all else fails)