86 GT Coupe

I have a 1986 GT Coupe and have the original 185/60/14 tires on the car. While driving recently, a passenger noted the speed on the dash (I have a needle not an electronic dash), and said we were actually going much quicker. Didn't believe it at first, but got a GPS and checked it. Turns-out that at anywhere over 50 MPH or so, I'm actually going about 8 MPH quicker. So when the speedo says 60 MPH, I'm actually doing 68 MPH.

Anyone know how I might be able to fix/adjust this? The speedo was replaced 25k miles ago (when the car had approx 110k on it at the time). I have also noticed every once in awhile that right around 60 MPH or so, that the speedo actually jumps from say 60 MPH to the actual speed of 68 MPH. This happens infrequently, but when it does "jump" (the cars actual speed doesn't change) I do get the correct speed read-out. Any help on fix would be much appreciated.

ddg=shinebox racing

I went through the same thing, I swapped speedo's with one that had a working odometer, it read about 10mph high, I then realized I had pulled it from a 4000, they have a different final drive ratio!
I fixed my old one and reinstalled it!!
Even the stock speedo reads 8% high, I'd put 195/60-14's to compensate and make it more accurate!