My 78 I had to change the tranmission a year ago and the trans I put in now reads that I'm doing 15mph faster. Took it to the tranny shops and they put in a 19 gear and it still is out. The ckuster and cable and gaer are good. They say the splines that the gear matts onto in probally wore and needs to be replaced. This would require them to tear the trans tailshaft apart to replace the splines on the output shaft thatthe speedo operates from.
So I have not done this. So on the highway my speedo says I'm doing nearly 80 and I'm actually doing 60. For now I'm leaving it that way.