Since the output shaft of the transmission has a metal gear on it...and the gear on the speedo is plastic...they tend to wear out the plastic speedo gear (apple core look)

This happens especially when the output shaft bushing gets worn out...and output shaft moves around a little more than it should....or the speedo housing gets worn out (speedo gears are sometimes made from teflon..especially replacement ones)...if you have a speedo gear housing leak..this is something you need to fix when replacing speedo gear....replace the housing if the shaft bore is worn out...

check for leak at output shaft while you are under the unit...this is a related issue that can happen if bushing is worn out...

if the speedo gear teeth looks good...dont forget to pull the gear and make sure the square hole where the cable slides into it is not chewed up (end of cable should look nice and slide in nice and secure into the gear)...I have seen that happen also....

other possibility is the speedo gauge itself has a broken/worn out recess where the cable slides into the back of it....this is a pain in the azz to check....

speedo gauge might be broken...but unlikely since I have never seen this...

check the cable like I recommended in my earlier post.....when you pull the gear housing out of transmission tail...spin the speedo gear by hand and have someone watch gauge to make sure it is working...needle should jump a little when you spin the gear