[Ski-me]

Start with vacuum hoses to be sure everything is hooked up correctly and not leaking. Because of the age, it might be good to try and swap in new tubing anyways. Check the plunger too. Maybe that is leaking or not working correctly.....and also check the amount of slack from the plunger to the TBI unit. If I recall correctly in Chiltons, there should only be about 1/8" - 1/4" slack and almost tight. If you have a lot of slack, maybe this is where your speed is dropping so much. I've not heard of a speed drop that much after setting. Mine use to drop maybe 1-2 mph but that's it.

The correlation between the speedo and the cruise is the DRAC box. It converts the signal from the speed sensor into pulses and then sends it out to various devices separately. So if the speedo ground is disconnected, the DRAC still sends out a signal to the ECM, Cruise and ABS.

I converted my 89' cable speedo to the 90-91 speedo and it works great now! I did have to re-pin the cluster connector but now it works awesome! I also placed a 7 pin selector in the DRAC to accomodate for larger tire size and axle ratio. 35" tires and I'm dead on with my speedometer reading.