Is there a way to calibrate the speedometer in a 98 Catera? Mine reads about 10 MPH lower then I'm going at a true speed of 60MPH.

Hmm is the catera a cable speedometer system that connects to transmission if so it could use a little lubrication

Most cars stopped using the mechanical cable in the mid 80's. The catera does not use one.

What is the tire size currently on your car?

Im pretty sure you would need some 40" mudders to set the speedo off by 10 mph.

Not sure where your problem might be, how are you checking your speedometer accuracy?

The system uses the right rear wheel speed sensor, reporting to the EBCM, which in turn generates a signal that goes to the PCM and then I think to the IPC for display.
Do you have the correct size tires on the car?
It is not "adjustable".