Speedo not right.
 

I'm not sure if this is the right forum for this.

The speedo does not read right, your going fast than is says. It's a 1983 380SEC that has a motor, tranny and dash from a 1983 300SD W123.
Not sure how to correct this issue.

By changing the Dash, I assume you mean the cluster was changed from the SD. If the Differential is the SEC original, then it is a 2:47 ratio. the SD used a 3:07 81 - 84 and a 2:88 in 85.

The Spedo and Diff are geared together. all MB trannys are 1:1, except for the 5 spd manual with OD.

Look at the Diff. on the lower left corner. scrape off the gunk and some numbers will show. the last 3 are the ratio.

Charlie

Hmm
 

There never was an 1983 300SD W123.

Did you mean a 1983 300D W123 ??

Or a 1983 300SD W126 ??



.

The guy I got it from said it came out of a 300CD W123 1983, then told me he was wrong it was a 300SD... That's what I have to go on.

OK
 

Just FYI

A W123 gauge cluster will NOT fit or function in a W126 chassis .

It probably is from a 300SD = W126 chassis.

Here is a list, if you are interested.
US Model Chart to 1946

I think I have an SEC cluster on my bench... it may be geared for 227 though...