Could be that your calibration point got changed. Turn the key on and press the speed button. It will display a calibration number. The operators manual will tell you how to set it up correctly. HTH.
Ground speed originates from a reluctance sensor located in the top of the transaxle. Check all the wiring for loose connections between the axle and the instrument cluster. Might also might take connectors apart looking for corrosion. Then reassamble using di-electric compound.