W123 Diff change and speedometer question - PeachParts Mercedes-Benz Forum

79Mercy · #1 06-07-2009, 07:39 PM

I wantt o swap out my 3.58 diff to a 3.09. I feel like im burning up the motor at 70MPH even though im sure its fine. A 3.09 diff would lower the engine speed by about 600 RPM.

So will I have tyo swap out my speedometer for one that works with a 3.09 or can it be adjusted to read properly?

Thanks

barry123400 · #2 06-07-2009, 08:09 PM

This has been bothering me a little as well. I want to install a 2.88 rear end from a 1985 but their speedo is electric. Except for the station wagons perhaps. You need a compatable speedometer from the donar vehicle in my opinion to give accurate speeds. I am not aware if a speedo shop can recalibrate our older speedo heads. It's either that or memorise a correction factor in my mind.

::matthew · #3 06-07-2009, 08:31 PM

Quote:

Originally Posted by barry123400

This has been bothering me a little as well. I want to install a 2.88 rear end from a 1985 but their speedo is electric. Except for the station wagons perhaps. You need a compatable speedometer from the donar vehicle in my opinion to give accurate speeds. I am not aware if a speedo shop can recalibrate our older speedo heads. It's either that or memorise a correction factor in my mind.

An 85 123 speedo is not electric.

TheDon · #4 06-08-2009, 10:40 AM

Quote:

Originally Posted by barry123400

This has been bothering me a little as well. I want to install a 2.88 rear end from a 1985 but their speedo is electric. Except for the station wagons perhaps. You need a compatable speedometer from the donar vehicle in my opinion to give accurate speeds. I am not aware if a speedo shop can recalibrate our older speedo heads. It's either that or memorise a correction factor in my mind.

the tach is electric, not the spedo.. the W126 has the electric speedo

79Mercy · #5 06-07-2009, 08:13 PM

Thanks.

What am I loking for, a 3.07, 3.08, or a 3.09? I forget which one is used in 300D's, what year 300D?

t walgamuth · #6 06-07-2009, 08:14 PM

307.

79Mercy · #7 06-07-2009, 08:18 PM

thank you. That came in the pre-1985 300D turbo's, correct?

79Mercy · #8 06-07-2009, 08:34 PM

I want to say that the 83 300TD speedo is though, correct?

barry123400 · #9 06-07-2009, 09:43 PM

Thought the first year of the electric speedometer was 1985 123s.. Glad to be wrong. Maybe I was thinking of the 126 series?

I was pretty sure all prior to 1985 123s where mechanical anyways. Any 123 turbo with a 307 will be mechanical I suspect. So you should be good to go.

My 1977 300d with a 3.47 sounds a little busy on the highway but the two 616 models with their 3.69s are worse.

DeliveryValve · #10 06-07-2009, 09:45 PM

Quote:

Originally Posted by 79Mercy

I want to say that the 83 300TD speedo is though, correct?

That would be the 1981 300TD that had the electric speedo.

Quote:

Originally Posted by barry123400

Thought the first year of the electric speedometer was 1985. Glad to be wrong. Maybe I was thinking of the 126 series? I was pretty sure all prior to 1985 123s where mechanical anyways. Any 123 turbo with a 307 will be mechanical I suspect.

All 126s had the electric speedo.

barry123400 · #11 06-07-2009, 09:53 PM

Actually this is great news. Now the hunt starts for a 1985 differential and speedo head. I want a 2.88 rear end for one of my turbos.

Squabble · #12 06-08-2009, 06:38 AM

just to clarify, if you want to update the diff, you have to match the speedo out of the same donor car/year, correct? i thought i saw a thread by deliveryvalve showing how to adjust your speedo to avoid this, if it is a problem... i too want the diff and speedo out of an 85 300D someday.

79Mercy · #13 06-08-2009, 08:21 AM

I would use a 2.88 but im afriad my car wold be a dog off the line. My car has about 25 more HP than a 85 300D.

::matthew · #14 06-08-2009, 08:42 AM

Quote:

Originally Posted by 79Mercy

I would use a 2.88 but im afriad my car wold be a dog off the line. My car has about 25 more HP than a 85 300D.

how does the torque compare? That's probably a better indicator of how it would do off the line.

charmalu · #15 06-08-2009, 11:31 AM

Quote:

Originally Posted by 79Mercy

I would use a 2.88 but im afriad my car wold be a dog off the line. My car has about 25 more HP than a 85 300D.

Going from a 3:58 to a 3:07 wil be a jump down from off line performance. a 2:88 will drop it a little more.

I have the 2:88 with the 4-spd manual, It isn`t all that bad. now if I had the 3:58 and going to a 2:88 I might feel different about it. MPG wise I would want the higher geared Diff. with the 4-spd I have been getting in the 27 to high 30`s. quite a jump from the automatic.

all depends what you want, performance or milage. also depends on the age group of who wants what. with fuel costs only to go higher, I prefere milage.

using a drill and a spedo cable to advance the spedo that many miles, will take some time. I used my miketa cordless, and I think it only turned it 45mph.
I have a spedo I have taken apart to see how it works down to just the main guts of it. the number assenbly. it looks like if the shaft that holds the 6 grey little cog wheels that keep the numbers lined up was pulled out, the numbers could be turned to where you want them, then the shaft and cogs replaced one at a time, and the end of the shaft peened again.

Charlie