Kent recommends just sending it out for professional repair
Although it is possible to fix a speedometer yourself unless it is done precisely it may fail again in the not so distant future or may not be totally accurate. For the price (usually under $100) Kent recommends to send it out to get it done right. That is what he does with his own cars.
Problem & Solution
Common among these chassis:
W110 W111 W112 Sedan
W111 W112 Coupe
W113
W108 W109
W114
W115
R107
W116
W123
W124
W126
R129
W140
W201
Watch the youtube video and he will explain why and show you a good example of what is involved:
Contact information for Overseas Speedometer:
Rick Borth
Overseas speedometer, www.speedometer.com
100 Wallis Dr. Austin, Texas 78746
18004440292, 15123298200
Hours: Monday through Thursday: 9-5pm