Depending on what car.
The Mercedes w123 mechanical and w126 electronic speedometer calibration is done on the speedo head itself.

For a w123 with a 2.88...

You'll need a speedo head with the part number 123 542 27 57 on the face found on the '85 300D

Or if you want to use the original speedometer, then a Ratio Adapter box spliced in the cable will need to be done.
Like this product..
http://www.nhspeedometer.com/


For a w126 with a 2.88...

You'll need a speedo head with the part number 126 542 81 01 on the face found on the '85 300SD

Or if you want to use the original speedometer, then a electronic signal interface would be needed.

Like this product...
http://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=126/category_id=287/home_id=59/mode=prod/prd126.htm





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1983 123.133 California
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