1998 Montero Sport Speedometer Problem

My Montero Sport's peedometer shows that I am going about ten miles an hour slower than I really am, I took it to the dealership and they said that the problem was a speed semsor or gear transfer case they also said that I had larger tires than what was supose to be one the vehicle but they said that that should actually make the spedometer show a faster than mornal speed.

What should I do and how can I fix this problem?