Speedometer issue

[mellowyellow598]

My father and I a while ago began to spruce up our '88 Sierra. One of the things we did was put in a newer Z71 guage cluster off a '94 so we could have a tach. Obviously the first big problem was that the wiring harness was completely different. Using a wiring schematic, we managed to hook everything up, even the tach. Everything works perfectly, except for the speedomter. Initially, we would get no signal at all, but after looking at a '94, we realised we were missing a key component. We did get the white box (I beleive this is the ECM, but I can't remember for the life of me what it is called) and wired it up as well. This gave us a signal, but the readouts are way off. Just setting the wheels into motion makes the speedo jump up to 10mph. Rolling out of the driveway we are traveling 60mph! The best we can figure is that the resistance in the wire must be different than the stock wire (less, judging from Ohm's Law?), causing it to recieve too much signal for a given speed.

So, any thoughts? I would appreciate anything at all.

Thanks.