My odometer doesn't work but speedometer does, what could be wrong? (97 Mustang)?

I bought this car a while ago and the miles are stuck at 177,274 and neither the trip nor the regular odometer works. At first I thought it might be a speed sensor, but I don't know if that would make sense because the speedometer is working fine. I am going to take the instrument panel out tomorrow and try to see what I can do but until then I thought I would get a few suggestions as to what might be the matter.