Your odometer is probably just broken, its not that uncommon with vehicles that have the mechanical style. The speedo and odometer use the same electrical signal input from the VSS. You will have to replace the speedometer assembly which comes with the odometer built into it.
It's real easy to do. Just take your time. I never disconected the battery or air bags to remove the cluster. It's about an hour job the first time, a little bit longer for me to stop and take all the pictures.
2 months after fixing it, Mom traded it in on a yellow '06 GT convertable
Wanted to use the smiley w/the guy giving you the finger but that must be on a different forum. FL is a big state dumb azz!
He gives you both gears. The gears disentegrate from the grease that was used. One might look good but I would not trust it. Replace both gears and you won't ever have to do it again.