False Mileage Reading on GPS Magellan

[turtle0747]

When we are on the turnpike at a certain point, we get a false milage reading. It will say we are only 10 miles away from home, when we are actually about 90 miles away. After traveling about 16 miles it will go back to the true reading. It is all ways between the same mile markers. Does anybody have any ideas on this? It is a Magellan Maestro 3100.