mileage change ??
 

I have a '99 base Boxster, with 52,689 miles. left the key in the ignition while cleaning it, ignition was off. Came back later and the mileage and speedo was lit up and had changed to Km, reading 99999. When I changed it back to MPH the mileage jumped to 59999. I disconnected the battery for an hour came back and no change. What do I do to get the correct mileage back?