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.

79Mercy · #3 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 · #4 06-07-2009, 08:14 PM

307.

79Mercy · #5 06-07-2009, 08:18 PM

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

::matthew · #6 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.

79Mercy · #7 06-07-2009, 08:34 PM

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

barry123400 · #8 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 · #9 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 · #10 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 · #11 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 · #12 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 · #13 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.

79Mercy · #14 06-08-2009, 09:12 AM

Matt, I'll have to get Alfie(thebenzbar) to drag out MB his books and check for me.

79Mercy · #15 06-08-2009, 10:30 AM

Anyone know if you can advance the mileage on a odometer? I need it to read about 214,500. The one I could get has 150,000 on it.