Information on how the speedo circuit is wired would be very helpful in diagnosing your problem.
I suspect that the Autometer speedo is not set up for the 24000ppm or 4000 ppm signal from the sending unit. (depending on what transmission used)
You will need to purchase a converter to allow the two units to communicate.
They are available from many different vendors, possibly Autometer can point you to the correct unit for you application.
[This message has been edited by olejoedad (edited 02-08-2019).]