2005 Chevy Impala SPEEDOMETER DOES NOT WORK ALL THE TIME

Electrical problem
2005 Chevy Impala 6 cyl Wheel Drive Type unknown 97340 miles

MY SPEEDOMETER DOES NOT GO ALL THE WAY BACK TO ZERO ALL THE TIME IT STAYS AROUND 50 TO 70 MILES PER HOUR SO I DONT KNOW HOW FAST IM GOING WHAT COULD BE THE PROBLEM