Better gear ratio for W123 Diesel – What axle would fit ??
 

15" wheels should help the MPG on my 300TD I mounted 15" steel rims with 205/75 tires. The RPM dropped by 10% so I hope I get some better than 20MPG. The ride is super nice on the highway and the tires are stiff enough for stability.

1. What differential could I use for better gear ratio??
2. In what other models can I find that differential ?

2.88 that was used on the '85 w123 and w126. You will need the '85's speedometer as well.

What ratio does my 1981 300TD wagon have now?

I'm guessing 3.07. It is stamped on the bottom of the rear diff, I just do not remember if it is the left or right corner. You can see it just below the rear diff cover. Of course it needs to be cleaned to read it.

If you're getting only 20mpg then you're either driving it VERY hard or there are problems dragging down its efficiency.

Search the forum for suggested maintenance points.

I don't think mpg is as related to rpm on a diesel as it is on a car. It takes a set amount of POWER to move the car x distance at y speed based on the rolling resistance of the tires and aeordynamic drag. That power requirement when factored with the efficiency of the engine gives fuel economy.
Increased resistance in any way (dragging brakes, added roof racks, who knows what else) will decrease economy, so will decreasing the efficiency of the engine with things like bad thermostats.

The same diesel engine will use more fuel running 3000 rpm vs 2500 rpm, going the same mph.

All the other stuff stated will also factor in.

I couldn't imagine only getting 20 mpg:eek:.
I think the worst I have ever gotten is around 25 in town
with a lead foot.

I think I would look into the maint. things too
Brakes dragging, injectors, tire pressure, etc.

The efficiency on internal combustion engines is highly depended RPM. If to low the efficiency is bad and if to high the efficiency is bad. I commute 35 miles each way and a speed on 75-80 MPG. The engine refs 3000-3500 RPM.

Next week I will try my MPG going no faster than 65...we will see.

Still hoping to find a 2.88 axle ....would a 300SD axle fit?

not sure this works out, at least not for me. I dropped my RPMs by a whole gear and I gained 2-3 city(so far) 5 hwy(estimated due to past difference, I will know when the snowbirds go home) MPG going from the 3.07 to the 2.47.

the 2.47 I have is from a w126, you just have to flip the rear mount. also, when you pull the rear make sure you keep the axles shims as they are sized to the diff.

you need to calculate your effective gear ratio. if it actually reduces rpm 10% then the actual mileage is 22. likewise the speed will be higher so if it reads 60 the actuak would be 66.

the 81 is na so the diff would be 346.

Look in the resources tab above and click on DIY articles. A good write up on R&R a Diff.

See Post #2 above for 2:88.

Charlie

For the 300D/CD, yes. The 300TD was the first non-300SD turbo in 81 so it should be 3.07.

Actually, according to my mb illustrated buyer's guide the cd was turob in '81 too. I wouldn't be surprised if it was a mid year change over.

I have yet to see a genuine one.

There were no 1981 model year 300CD turbo diesels. All 1981 model year 300CDs were normally aspirated. Around 8/81 or 9/81, production switched from non turbo to turbo 300CDs, and while they made a few hundred 300CD turbo diesels in late 81, they were all 1982 model year vehicles.

My 300D was assembled in 10/81. It is classed as an 82. It has the turbo.

I wrote this to respond to forced last night but forgot to hit submit apparantly.

Your example has NOTHING to do with the rotational speed of the engine and EVERYTHING to do with the force needed to push the car through the air. Force required cubes with the square of speed. Less speed = less force, less force = less fuel. Your mileage will rise significantly at 65mph vs 75-80 mph.

My '81 TD is most fuel efficient at about 50-55 mph--that's a little unrealistic for highway driving. Nevertheless, speed has a significant impact on mph.
RPM's definitely impact mileage, diesel or gasser--you can sit in your driveway revving to 3500rpm for 10 minutes and burn a lot more fuel than at normal idle.

On your picture it looks like you have bigger wheels as well. How does the 2.47 drive and from which car did you get the axle???

I LOVE the 2.47. The only drawback is my speedo is off by 40%. also the wheels are large but the tire OD is actually smaller so that makes the 2.47 a little higher. I have a manual trans so an auto probably wouldnt notice the 1st gear takoff sluggishness I have. once I get to 5MPH first is like 2nd gear used to be and 4th is like an overdrive

I was watching my speedo closely with regard to RPM, this is what I have. At 45MPH (corrected to 63MPH) I turn 2200RPM. I think I have mis stated this in the past because I was guessing but this is the right numbers verified today. 4th has all the power I need to climb hills and will accellerate just fine. If I need to pass 3rd is the same as 4th used to be

I took the 2.47 from a 380 or 500 sel I cant remember.

Does the flexdisc fit from your car? I know that on gas cars different sizes are used from V8 to I6.

I removed the 3 hole flange from the back of an old transmission I had and used it on the 2.47 from the SEL. I guess you can also use the one from the rear you remove but I wanted to keep mine intact in case I didnt like it or I wanted to put in a five speed

Is there anyway to correct that?

I am looking at correction devices, unfortunately there is no cheap solution. there are mechanical ones with set ratios around $80, but Im not sure they will screw on and there are electrical drivers that are much more expensive but infinitely adjustable and again not sure if the threads are the same. I do the calc in my head for free

You can always get the speedo from an 81 300TD. Since its an electric drive (the only one in the W123 line for some reason...) it should be much easier/cheaper to alter it.

in all of my yard prowls to date, I have never seen an MB wagon. what about a later model speedo after 85? arent those also electronic? has anyone made a sender for one?

You can try to modify a 380SE speedo to fit in your car. Forum member Apotheoun did a version of this with his 300SDS had it done with his 4 speed conversion with magnets.

yeah, not sure this will work for me, the 300CD uses a mechanical pickup for the cruise control and I would rather not lose that.

Your statement ignores the point of the question and fact that the efficiency of an internal combustion engine is dependent on RPM. Try to go cross country in 3rd gear .
My attempt is to optimize MPG at a given commuter speed of 75MPH. This speed is probably higher than what the vehicle was optimized for. (Probably 60 or something).
Since a 5th gear to lower RPM is out of question, the only remaining option is the overall gear ratio. After more than 3000 miles I know now that 12% taller tires help about 2-3%. on my MPG.

Did you correct your calculations for odo and speedo error caused by the taller tires?

Different gear ratios between sedan vs. wagon?
 

I have never read up on the various differential gear ratios in the 123s, so reading this string is interesting. I have a simple question....

In my previous 85 300DT (sedan), I could get highway mileage of 30 mpg as long as I kept speed between 60 and 65 mph. In my current 84 300TD (wagon), I can only get 25 mpg. I realize these are two different cars, and various differences may play a role (one car may have better injector spray pattern, better compression, etc.). But if we assume all other factors being equal, is the lower fuel economy in the wagon due to different gear ratios? E.g. is the wagon geared lower because the car is designed to haul heavy loads?

Thanks,

Mark

All 1985 models got a 2.88 rear end while 1984 and earlier turbos got 3.07.

Sedan and wagon drivetrains are identical.

Before others flame you, you have a 300D. There is no such thing as a 300DT. All 300D's made after 1981 are automatically turbo except for special imports.

Yes , I recalculated the difference in odometer reading - additionally I compared the MPG commuting the exact same trip I take since 11 years : 35 miles one way and 350Miles a week.

We need a full write up thread with pics on this! I may be moving in that direction with my sonic wagon.

I put the 2.47 in my CD, along with a 4 speed. I love the more relaxed RPM, much better mileage, and quieter rpm on the highway. I can't comment definitively on my mileage improvement yet, but on the first half of my road trip, I made 35mpg!

I wouldn't do it unless you do a good bit of highway driving. If you do though, the swap is well worth it!