Originally Posted by only in a jeep cj
The speedometer is a set calibration from the factory and requires much know how to adjust it, but that is not the correct solution in your case. As said, the speedo gear in the transfercase is the solution. It is under where your cable screws in to the rear section. 1985 with a T5, you most likely have a 31 tooth gear and will need one a few teeth less so it spins a tad faster and bumps up the speedo.
I believe this same place sells a adapter that has different ratios. While more expensive ($65) they do work, its installed on the TC externally, the cable then screws into it. They do work well, I have 2. One of them has 40,xxx miles on it.
They are avaible in different ratios.