If your car is leased, get it fixed pronto (stop driving!). Otherwise, you'll be knocked with a "Total mileage unknown" and will be charged the max they can get away with for you going over the allocated miles on your lease.It could also cause problems if you try to sell or retitle the vehicle, as you cannot give an accurate odometer/mileage reading.It could be something as simple as the cable going to the odometer became unplugged, or it may just be a defective odometer... But you'll have to see a dealer (and you shouldn't be adding miles to it in the meantime) to get it fixed. If it's a digital odometer, the correct mileage may have to be burned into the computer chip by the manufacturer...