I would think your Speedo ratio is WAY off compared to your tranny drive gear...Mine 350/350th...is off bought 12 mph and I need to adjust the teeth on the tranny drive gear, But burryin the speedo at 10 to 15 MPH seems like something is way outta wack and changin the tranny gear will not help.
I bet ya need to replace the speedo.
Tim


1952 Chevy Shortbed

Photos in Photobucket

If It Aint Broke Fix It Till It Is!