The 91 cluster has the digital ratio adapter controller built into the cluster itself.
There are jumpers in the cluster which tell the DRAC what your tire size and gearing should be (hard wired into the DRAC by the OEM). It takes this information and looks at the pulses from your VSS (vehicle speed sensor), converts it to a signal the speedo can use and then routes it to the speedometer which displays your road speed. It also routes separate signals to your ECM, Cruise control and ABS so they also know the road speed for things like maintaining cruise, locking up the convertor, shifting the transmission, etc.
What has likely happened is you swapped in a cluster from a truck with different gearing, so the DRAC is outputting an incorrect signal.
You need to recalibrate the DRAC in the cluster.
It is slightly harder than the later models that have an external drac because you have to open the cluster, but it can be done.
There was one model that had to use a "shorting plug" and it was pretty much a one time calibration thing. It worked by shorting certain resistors/circuits to calibrate the cluster. That was right around the 91 model year IIRC. Memory is a little foggy on which one those were, you'll have to google a bit if you want to know for sure. Here's a link that seems to indicate 91 and lower were shorting plugs:
http://books.google.ca/books?id=sjd...ac digital ratio adapter converter 91&f=false
I'm pretty sure guys have figured a way around that and use the DIP switch scheme, but again, you'll have to google.....
If you alter the DRAC circuit with DIP switches vice jumpers, your DRAC is easily adjusted when you do things like change gearing or tire sizes. Not as easily as an external DRAC (VSSB in later trucks), but still easier than de-soldering and moving jumper wires.
You can also use the switches to dial your road speed dead on with another source (GPS for example).
