Differences in factory speedometer internals??

[Kaysenmc]

Here's the story.. About a week ago I was going to lube my 87s squeeking speedo cable and removed everything needed to get to the speedo. I always hate trying to unhook the cable from the speedometer head with such little space, so I unhooked the cable at the trans and fed the cable through the firewall for a little more working room. I noticed that the cast aluminum housing looked like it had some really bad casting marks (turned out to be cracks) on it on all sides. I proceeded to unhook the speed sensor (5.5mm bolt, I believe) then the cable, when the cast housing busted into probably 30 pieces. Gears, face, and odometer hit the floor but the part the cable hooks into was still stuck in the end of the cable.
So I have an 84 lwb I have been robbing parts off of and yanked the speedo without the sensor to swap into my 87. My 87's speedo had a gray drive gear that runs (use to run) the odometer, but in the one outta the 84 the same part is light yellow and is shaped differently.
Here's the problem... My speedometer has read slow by ten or so miles an hour (at 65)with my current powertrain setup (tbi 350, 700r4 (I do not know what speedometer gears it has in it) and 3.40 gear rearend (swapped from 2.73)). But now with the 84 speedometer I am reading about 15 mph fast at 65.
Heres the question... are 87 model speedometers a one year only piece (I know the faces will NOT interchange without modification) or are the speedos calibrated for different geared rearends from the factory? Or do I need to swap the speed sensor too? I have searched locally and online but have come up empty...
Thanks in advance and sorry for the long thread!