The input end of the speedometer head is round from the factory, it becomes square down 3/16" or so, this is the way that they are made. Therefore you must make sure that the end of the cable engages the speedometer far enough to get into the hole to drive it, about 7/16". As Big Daddy says, there are 2 sizes of the cable ends and if you use the smaller cable in a oval head, it will work for a while until it rounds the corners off of the drive. Snyders offers a "special" cable that is mismatched, it is a square drive on the bottom and a large square on the top for those that have the wrong drive at the torque tube for the oval speedometer. You may also find that the cable is going too far down into the drive at the torque tube, making the cable short at the top end. I have solved this by inserting a space in the driving shaft to force the cable up where it belongs.