Yes this happened to my 1998 catera as well; fixed it myself:
put the front-tires up on ramps, turn car off & lock emergency brake
crawl approx. middle of the car- you'll see the drive column
locate a deppressed area and look for a black wire attached to a sensor perched above the drive column- I think its a 10mm bolt head. Unloosen bolt-head and replace sensor. Start car- the speedo-neddle will return to normal.