I swapped trannys from my cars. Both the same gears and same specs, gear ratio wise. But one car is over, and the speedo is about 25mph higher than what it should be. Any ideas?
Check to make sure the same color gear on the output shaft of the tranny is the same. I believe they came in 6 tooth (black), 7 tooth (yellow), and 8 tooth (green). I have had this problem before and it turned out I had the 8 tooth and I needed the yellow. Different years had different colors on otherwise same transmission. I had a link to a chart that showed the different combos, but it is dead now.
Did you use the Vehicle Speed Sensor (VSS) from the old tranny or just keep the VSS from the new(er) trans in there? The new(er) trans may have a different VSS gear calibrated for the rear-end gear ratio that was in the vehicle it came out of, not your rear-end ratio...
The tooth count is denoted by different colors... Ex: Red gear = 19 tooth for 3.73 rear gears (certain year t5s).
I found a chart in a catalog that has the different combos in it. A 1990 T5 should have an 8 tooth (green) gear on the output shaft. Pull your speedo sender out of the tranny and you should be able to see it. I'll bet you have a yellow one in there. Or you have the green gear but your speedo has the gear for a yellow one. My cars' speedo was way off when I bought it and that is what it was. You can either swap it out or just get the correct gear for the speedo. Tell me what rearend ratio and what color output gear you have and I'll let you know what gear you need.