Speedometer is reading 20 to 30 mph too fast. on 2001 Buick Park Avenue

I am ASE mechanic as side job.

Asked by for the 2001 Buick Park Avenue
make sure you have the correct tire size on the car.

otherwise, the trans gear inside the governor may be at fault.

The tires have not been replaced, it just started doing this about 1 week ago. I suspect the speed sensor is failing.