Speedometor problems on 2004 Buick Century

The speedometor works for awhile and then it jumps ahead and it looks like I am going 90 miles an hour when I am going 30 miles. How can I fix it and how much will this cost?

Asked by for the 2004 Buick Century
Chances are the problem is internal to the instrument cluster. Proper diagnoses will be required to confirm this. If the fault turns out to be the cluster it will need to be remove and sent to a shop specializing in cluster repairs. I am sorry I cannot provide an estimate for this repair.