I just swapped out the stock gauges in my 79 for some 86 gt gauges. It seems like it's accelerating a wee bit too fast on the speedo, does the 86 gt have a different speedometer gear than my 79? I have an automatic tranny, btw.
The speedometer driven gear is inside the transmission, on that end of the speedometer cable. Interchange speedometers all you like. If one seems to read different than another, one of them was ****ed up to begin with.