I have had 2 (19 teeth) speedometer gears go out in the last 2 months on my 1956 ford crown victoria with 292 and fordomatic transmission. The first gear came with the car which I have had about a year. The second was a reproduction speedometer gear from Tee-Bird Products in PA.  I had a retired mechanic friend install the new reproduction gear. Both failures occurred while driving approximately 65 MPH. Any ideas on what could be causing this? I also replaced the speedometer cable after the first gear failure.

Thanks,

Danny