Speedo Problem

[W0rp3D]

This seemed to be the closest thing to an electrical section so i thought i would post it here.

I have just installed an epson ej1 and nizmo dash with lcd screen, all went well apart from the speedo now says i am doing a little over half the speed im actually doing, i removed my avcr display when i put the dash in which i know was only 2k's out, so i plugged it back in to check the speed and now my avcr is doing the exact same thing.

The only thing i can think that i did wrong was when i wired it up one of the wires, it was not the actual wire off the ecu, there was a wire coming off the ecu wiring going to the avcr so it was easier to just use the avcr wire to tap into, im not even sure if it was actually the speedo wire that i did it with, any idea if this could be the problem, or any other ideas what it could be?