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:
Watch the youtube video and he will explain why and show you a good example of what is involved:
Contact information for Overseas Speedometer:
Overseas speedometer, www.speedometer.com
100 Wallis Dr. Austin, Texas 78746
Hours: Monday through Thursday: 9-5pm