I recently replaced my 280zx transmission. Now the digital speedometer reads 8 to 10 MPH less than the actual speed (I tested it against my GPS). Anybody know what may be wrong?

 

The speedometer cog in the transmission is set for a different rear gear.

Pull the cog out of your old transmission and install it into the new transmission. The cog is the device in the transmission that the speedometer cable plugs into. You can get it out by removing one little bolt.

 

Yeah what he said I had a simular prob. with mine it was a different color, and now the cable doesn't even reach.