3.07 / 2.88 diff swap???
 

After reading several posts re the diff swap from 3.07 to 2.88 on 1984 300 D turbo I find the question of fuel consumtion was asked a few times but never answered. Is there any info or reasoning to suspect the ecconomy would be bad in town and good on the highway, or would the fuel consumption be greater all the time because the load on the engine would be greater, in town and on highway. Could anyone who has done this swap put this question to bed before I rush out and pick up a new differential.

I recently pulled a 2:88 diffy from my parts car in order to do this exact thing.

But, I do not have empirical evidence to support any of this speculation on mileage gained or lost through final drive ratios. I do happen to have two remarkably similar wagons on which to base my decision to swap diffys; one with a 3.07:1 (Marlene-1983 300TD) and one with the 2.88 (Passion Flower - 1895 300TD). Unfortunately, the 'Flower's odometer has never worked but, she has always seemed to go further on a tank of fuel than Marlene.

My gut feeling is that with the higher ratio, in-town mileage may suffer ever so slightly or, remain unchanged while experiencing a marked improvement in highway mileage. Here's what I base my theory on:
Same or reduced intown mileage — Possibly unchanged because engine is consistently operating at lower RPM. Potential for slightly reduced in-town economy because engine is being operated at WOT more frequently to accomodate reduced acceleration due to higher gear ratio.

Inreased highway mileage — Changing to a 2:88 ratio will drop the RPM of the engine somewhere around 15%. If you're turning 3100 rpm @ 68 mph with a 3.07, you'll reduce cruise rpm to approximately 2700 rpm with the 2:88. This reduction drops the engine speed right into the big middle of the torque curve or, where it is operating at peak volumetric efficiency (highest economy).

Remember, the US speed limit was 55 mph when these cars were imported into the country. Diesels frown on operating outside their designed powerband so it makes perfect sense that the Germans would gear them for proper rpms (with the 3.07 diffy) at the, then maximum, highway speed of 55.

The drop in RPM will only be about 6.2%
 

if you change from 3.07 to 2.88. I'm at the other extreme -- I'd love to find a good solid 3.55 from a non-turbo to put in my 2.88 '85 wagon because the daily commute is all on roads which have now had their speed limits reduced to 45 or 35 mph and stop lights at what used to be country road intersections because of suburban sprawl exceeding the speed of sound without any matching investment in road improvements here.

Re 3.07/2.88 swap
 

Thanks for the info guys, I feel more confident now to proceed with the swap. In a month or so I will report back to the board with the results. I need to replace an axle anyway.
John

Same here! I cant wait to try the 3.69 in my 240! It will be a welcome change from my 3.07 while I'm in the 20mph traffic of I-25.

Here's the W123 and W126 ratio's-

W123
----------
230,240D RATIO 1:3.69 616 engine

280E RATIO 1:3.58 Gas 6 cylinder

300D RATIO 1:3.46 617-912 ie NA 79-80 TD's 77-81 C/D 77-81

300DT 85 RATIO 1:2.88 all turbo 85's

300DT TO 84 RATIO 1:3.07 all turbo non 85's

W126
----------
300SD 85 RATIO 1:2.88 ABS

300SD TO 85 RATIO 1:3.07

300SDL RATIO 1:2.88 ABS

300SE,SEL RATIO 1:3.46

380,500 RATIO 1:2.47

380,500,560SEL RATIO 1:2.47 ABS

420SEL RATIO 1:2.47

You guys that want steep rear gears for acceleration, I have a few MB gas cars at work with such things. I could probibly get one to you through work for a couple hundred, shipped. I have a 2.88, but me thinks I'll keep that for myself. :)

Those SOBs are heavy...I swear they're heavier than a manual tranny.

You want one? I have the 3.69 from my 240D for sale if you want. I never ran the car with it, so it's cheap for you, $50. It's not broken, but if it whines (242k miles), I'll take it back. And no shipping costs from Denver. :D

Damn these things last a LONG time. 110 years (1895-2005) and still on the road! :D

No thanks, part of the luck of having a 240D to start with is the 3.69 is already in! :D It's too bad you don't have the 3.69 still in, I wanted to GTG with you to see what a 3.69 with an OM617 would be like. :(

Don't get too excited about a fuel economy bump from the 3:07 to the 2:88. I can't tell much fuel difference between any of these three cars of mine:

85 300DT 2:88
84 300DT 3:07
82 240D with a 300 Turbo, 4 speed and a 3:07

They all seem to get 25-27 contingent on driving.

Don

To Wolf Walker:
 

It'll be a few months before I get finished with the conversion of my middle daughter's SAAB 900S automatic into an SPG with 5-speed (via an incredibly rusted out donor car) so that I have time and space for another swap, but I'll keep your message in mind and check back if you still have them available. After consulting MTUpower's list, I think that I might prefer the 3.46, if you have one then.

Wow Roy, you have a unique MB there, "(Passion Flower - 1895 300TD)"

...1985. :P