Will not pay. Come up with false excuses

I just had a problem with my transmission.  They said they would not pay because I had altered the vehicle tire status.  I purchase the vehicle new and it came with B F Goodrich whitelettered tires.  With which I have replaced now e times in 100000 miles.  I have kept the same size and kind of tire that Ford put on at the factory.  I have not changed a thing from the factory.  How can they say I have altered the truck?  Can not get anyone to talk to about this claim.  I am now indept to Ford for $994 after paying $2570 for a warrenty that is useless.  I just dont understand how they can get by saying this when the truck is equiped exactly like it was when Ford sent it out there door in 2005.