L8000 Speedometer

I have an '88 L8000 with the Ford in line 6 - not that that info has any bearing on the question I'm about to pose nor the answer. Briefly, the electronic speedometer was replaced some time back and never calibrated to the truck. The speedo has 8 DIP switches on its backside and the shop manual indicates they are to be set per specs in the"Parts Service Manual". Nobody at Midway Ford (local heavy truck dealership) has a clue as to what the manual is (or was) and can only say "bring it in and we'll see what we can do". Now I know little about solid state electronics, but I do know that at some point in time a spec sheet was available that would give the switch settings for the various axle ratios, tire sizes, and etc. Anybody out there old enough to remember what I'm talking about and, more importantly, where to find specifications? I have tried numerous other Ford dealers, heavy truck repair shops and speedometer shops to no avail...