Besides lubing your speedo cable, you also need a different speedo gear. If you know your rear end gear ratio and your tire diameter, you can select the right speedometer gear. It’s located where the speedo cable screws to the transmission. Check the tech archive under automatic transmission and you will find which gear you need. Sounds like your transmission is from a car with 2.94 gear and you have 3.55 gears. Which would create the situation you are describing.
I understand the speedo gear/tire size/rear end gear/etc but I do not believe that you can have a combo that would put you 100% off (my speedo shows ~double actual speed).
