3.69 vs. 3.08 vs. 2.88
 

I'm considering swapping rear differentials in my car.

I understand that my '83 240D, has a 3.69 differential in the rear. My top speed is about 80 mph without roof racks, its about 75 mph with a 4-bike+faring Yakima roof rack on top.

Switching to a lower number rear differential would reduce my top speed a bit, but also reduce the amount of fuel consumed, lower RPMs at cruise, etc. and result in increased fuel economy.

What would you guys say to a 3.08? What would you say my top speed would be, and what might I see MPG-wise?

Currently, top-speed of 75 mph is "nearly" WOT for me...although, I assume its not really consuming much more fuel because the engine can only spin so fast after all. Lower RPMs even at WOT = lower fuel consumption right?

Thanks in advance for the advice.

Youve got that backwards.. It will increase your top speed, thereby allowing your engine to turn at lower (possibly to low) RPMs.

Perhaps try a 3.58 3.54 or 3.46 first.

yeah, it will increase your top speed and make it even more of a dog off the line....ever try to pedal a ten-speed bike starting in 10th gear?

I hope you're not doing much travelling on the 408 or I-4 on ramps...

A 3.46 would make a noticeable difference but likely not hamper acceleration too badly. If you live in a flat, or mostly flat area close to sea level then a 3.08 would likely work as well BUT acceleration will definitely be affected. If you live in the hills forget it. I doubt either will make much difference in MPG since that 616 is giving all its got all the time already. It will make for a markedly quieter ride on the highway however. RT

Personally, I'd doubt it could ever get to the same top speed as before.

It will take the same amount of power to do the same amount of work, but once the transmission shifts to high, it's all up to the engine--which, running at lower rpm won't be producing the same power.

It's 100% all-the-time FOLLY to try to second-guess the engineers that designed the car.

If this is going in a 240D I doubt that a 3.08 or a 2.88 rear end will increase speed at all. It might increase MPG a little but I wouldn't consider it worth the drop in neck snapping acceleration 240Ds are know for. ;)

You may be able to accomplish some (but not all of) the same thing by using tires with a taller aspect ratio than what you have. You can buy a pair of taller ones and if you don't like the effect, put them in front.

The problem with the 240D (especially automatic) is that there is no available power. You can think about what you are doing on a 300D turbo etc...

I would hesitate to go any higher than a 358 gear which is only 3%. I feel you could pull that fine unless you have mountains to do on a regular basis. a 346 would be pushing it in my opinion but might work if you have pretty much all flat driving. Anything higher and you would not be able to keep it going on level ground in fourth gear, IMHO.

307 and 288 will simply not work, IMHO.

Tom W

I agree with Tom. The only thing you are going to get is a more comfortable cruising altitude (lower revs).

Hmm, fair enough, I won't bother then.

I'm not happy with the 22-ish MPG I'm getting currently, I know it can do better, but I'm running out of ideas on how to get there -- its had the valves adjusted, steering fixed, good new tires and alignment, only thing not done is injectors.

What do you expect to get for MPG? My 85 300sd gets 25-26ish and seems reasonable to me.

Given that you've got a turbocharged engine with an extra cylinder (or two?), I expect I should be able to achieve high 20s to about 30 mpg.

Not "35+" like the Craigslist ads claim, but a bit better than I'm doing.

Nope his engine isn't working as hard at say 60 as yours is, so I'm not surprised that it does better.

If you drove at 55, you would probably do better.

Sure, and I have never seen an updated part#. Or different setups for NA and the rest of the world. Or different rear diff ratios throughout the years, or in different markets. Or as options...

You are losing probably at least 3 mpg to the automatic tranny. If you were getting 25 to 26 running 75 with a stick that would be ok. Run 65 and you could touch 30 with a stick.

A stick conversion would probably reap your best dividends in mpg.

Tom W

I get 17mpg :)

I own an .82 automatic 240D that gets about 22 mpg city and at best, 24 highway. At least to reduce the racket at highway speeds, I would consider a 3.47 diff-swap, but nothing more radical. I may be able to try this, as I've been offered a free 80 300D with a bad tranny. Will post further if it happens.

Happy Motoring, Mark

In a car with big displacement and lots of torque, a taller rear wouldn't hurt you as much. A Vette can handle 1200 rpms in 6th gear because it has 6.0tlrs of torque. Your 69hp 240D can't. You might have a top speed of 65 mph if you go with a taller rear. If anything, as a previous poster suggested, try taller tires. If you're running 195,70R14's, try a 195/75R14 or a 205/70. If you're running 205/70's, try 205/75s.

I think you can probably get away with this with an automatic, but your acceleration getting up to highway speed will be glacial. Look out for the semis running you down on the on ramp!

Tom W

A 3.46 would be the best option. It will not hurt acceleration as much as a 3.07 or 2.88.

gee I know someone with a complete 4 speed for a W123.. he might be selling.. since his time machine needs a new fuel pump

wow... that sucks.. I'm getting 24mpg with my 300D turbo

turbo.. turbo... turbo

turbo
turbo


thats fun to say


turbo


tuuurrrrbooooooo

As my automatic 240D seems to rev higher in the various gears than my stickshift version (probably due to the inherent slippage in the torque-converter), I felt that with the 3.46 the tranny could compensate for some of the lost acceleration by hanging in the lower gears a bit longer.

Interesingly, for those wondering what axle ratios the factory intended for some of the different tranny options, while I don't have data for the 240D, one of my reference books gives the rear-end ratio for a mid '70s Euro 4-speed W114 230-6 as 3.46, but 3.92 for the same car with a 5-speed.

Happy Motoring, Mark

3.92- 200D, 220D, 200/200T
3.69- 240D/TD, 230E/CE/TE
3.58- 280E/CE/TE, 250/250T
3.46- 300D/CD/TD non-turbo
3.07- 300D/CD/TD/SD Turbo
2.88- 300D/CD/TD/SD Turbo, 85 only.

500ft pounds vs what about 70? In cars that have a pretty similer weight.

Maybe in a flat part of the country but around here we have hills, some are pretty steep. I would imagin a stock 240D would have trouble with some of them. Throw a high rear in the thing and you will be in 3rd or 2nd with your foot to the floor trying not to get rear ended.

It seems as if my automatic 240D first-gear is like a granny-low, but the engine runs out of steam in third, while trying to accelerate uphill at around 40 - 45 mph. In theory, with the proper rear-end ratio, I could take local hills in second, while third gear could provide the same freeway uphill performance that 4th does now, and 4th would serve as overdrive for flat and downhill use. But I don't know if running the automatic like that would be harmfull. In any case I may decide against the diff change as the 240D serves ideally for most of my local driving, while my Nissan is the primary long-distance car, and returns 30 - 35 highway mpg.

Happy Motoring, Mark

I don't think that will hurt the tranny. For a little more money though a stick transplant might be a better overall choice.

Tom W

Well, the 3.46 diff would be free and a much more simple swap than a tranny conversion, and the cheapest stick 240D parts-car I've seen in the past year was $350, for a very rusty 300+ mile example. Of course if I could find a suitable parts-car with a 5-speed....

Happy Motoring, Mark

Toms advice is probably pretty accurate. He has driven enough 240ds over the years.
That said I would check your injectors out as they could be the original units untouched all these years. . Also the pump timing and chain stretch. You really want every last horseower potentially available. I even feel this way about my 240ds with standards when I get a chance to use them.

BTW, it's 3.07 not 3.08.

I wouldn't put taller gears in a 240D if you ever need to drive up hills.

If you can put a 190D 5 speed in a 300SD could you put the same 5 speed in a 240D?

I wonder if that would be a better solution for you rather than playing with the rear end.

You cant put a 190d 5speed on a 617

I certainly have not found this to be the case... My 300TD with intercooler, adjusted/altered injector pump, upgraded injectors, non-stock timing, 2.47 vs 3.07 rear end, upgraded front suspension components, among other things, is a far better vehicle in nearly every respect than the original car... It is faster, smoother, quieter, and more efficient, while maintaining the same level of reliability. Just because they are "engineers" doesn't mean they designed a perfect car. They designed vehicles that did well with a 55mph speed limit. For an 80-90mph practical car you need to alter the gearing. Mine does quite nicely at about 90mph.. smooth, quiet, about 3200rpm.

With the 240D you are strapped for power, so going to a taller rear is probably going to hurt your top speed and decimate your acceleration. As stated before, you might realize some microscopic gains by switching to a 3.46 (and maybe even measurable gains with a manual 4 speed or better yet finding a 5spd trans, but not from a 190D), however, if you really want to get good mileage, you should look into modifying the aerodynamics of your car. Thats why you need horsepower. Making less HP = burning less fuel = better fuel economy. If you reduce your need to make HP at speed, you save fuel.

If you want more speed out of a 240D your best bet is to buy a turbo charged 300D. Same car, twice the HP.

:o

I stand corrected. I must have been thinking of some one that had an SDL that did the conversion then.

Any way you should be able to put a 300D euro 5 speed into a 240D then.

I don't really see the point of a 5-speed either. It would be useless on anything other than perfectly level ground and it probably wouldn't increase the top speed anyway. If you are driving on the highways, a 240D isn't the best car to buy.

Originally this thread was about improving mileage of a 240D and not increasing its top speed. A 5 speed might improve mileage.

I originally bought the manual-shift 240D as a parts-car, because I was considering converting my nicer-looking automatic 240D to a manual. Also my automatic 240D was experiencing low oil-pressure problems, so I was considering swapping engines as well. However the automatic car's oil-pressure problem seems to have subsided, and the 4-speed car runs so well that I decided to go ahead and repair it's floors and keep it as a second car at my parents home in Virginia Beach.
As for swapping the rear-end, it would be a relatively easy change. I actually have a spare 240D manual tranny, but a conversion would still be a major project, requiring a donor-car to supply flywheel, driveshaft, shifter, clutch hydraulics and pedal assembly, with all the extra labor. And If I decided the 2.46 rear wasn't up to my expectations, it would be relatively easy to change back. Besides, If I get the 300D parts-car for free, there's the potential to upgrade engines and make mine a 240D-3.0, while retaining the manual heat/AC which was one of the things that drew me to the 240D in the first placce.

Happy Motoring, Mark

Maybe, on the occasions when you could actually use 5th gear (downhill?). I seriously doubt there would be a reasonable payback time for the project if you were just looking for a couple of mpg.

MB seems to disagree since *most* 5speed cars I have seen are 240D and not 300Ds.

how much of an overdrive do they put in a 240D?

I have a german language owners manual that came with my euro 300d that I parted out. It shows the five speed as an option in the 240d. Remember the euro 240s have a little more power and are lighter. I believe the five speed is the same in all 123s in which case it has a 17 or 18% OD ratio. I believe that you would be fine on level ground in a 240 with the od and even probably on a small hill but on a bit more you would have to go to fourth.....no problemo in my mind.

Tom W

Ya OD is ~.83 iirc

Still wish I had one in my car :)

So, that would be the equivalent of about a 3.15 ratio. That might work on level ground, but it seems like you would only gain about 2-4 mpg when in 5th. I can't imagine the average would be more than 1-2 mpg for most people, it hardly seems worth the trouble/expense (until the cost of diesel hits about $10/gallon).

It might be hard to justify in dollars and cents but the lessened wear and noise would be good too. And the value of the car would be increased as well.

Tom W

It might be an interesting project. I really doubt the value would be increased to the general public, it would have a very small market (even smaller than a correct 240D). Personally, I never really use the 240D for highway trips and I still wouldn't if it was geared a tad higher, it's really just an "around town" type of car.

Oh believe me there is a strong market for the 5 speeds...

My 240Ds are primarily used for local driving, though my automatic version does seem to run quite happily (and noisily) at 70, possibly because that's the speed where the diesel generates it's maximum 67 horsepower.


Since the Euro W123s were never officially imported to the US, there seems to be very little technical information availble on the 5-speed option for those cars. It would be interesting to know if something other than a 3.69 rear-end ratio came in the Euro 5-speed 240D.
My only reference givin a ratio difference for Euro models, lists a 3.69 ratio for 4-speed W114 230-6 sedans, but 3.92 for the optional 5-speed. Obviously the factory wanted to compensate by adding back some performance in exchange for the additional gear.

Happy Motoring, Mark

Nope. AFAIK all 240D, no matter their origin, were 3.69.

Even with the Euro 5-speed option?

Happy Motoring, Mark