I have the same problem on my 73 dart. I swapped in a couple speedos and its still off the same. I used the correct gear for 3.21 and a 26" tire. Tried a gear with +1 tooth and -1 tooth and its not right with any of them. Mine shows 58 mph when its doing 50.
I think these things were never accurate, we just didnt have gps to check them.