In my case the GM navigation system tells me that a 250 mile trip is going to take 6 hours. My Garmin GPS and Google maps calculate the same trip of 250 miles will take about 4 hrs. It takes me about 4 hours and 15 minutes to drive the trip.
If you are having a similar problem go to your dealer and ask for Dealer Bulletin #PIC6075A. This is a known problem on some Buicks, Cadillacs, Chevrolets, and GMC. GM claims this is a normal operating characteristic. Does not mention having a fix for the problem.