I just went to the junkyard and got a new gauge cluster for my xj since my guages were broke. it had 255xxx on it so i took it apart and put my little miles thing in it and when i put it together i turned it on and the speedo shoots up to 15mph at idle. i had to take the needle off to get to the miles thing so idk if i put it on right. i know it worked fine before because i had it in my jeep before i switched the miles thing. so what do i have to do to get the speedo to be at 0 at idle. is there a trick of some sort?

 

Maybe try pulling the needle off, turn the jeep on and put the needle back on setting it at 0, then take her for a spin around the block to make sure it moves correctly