Routing works well, distance to destination is good, HOWEVER, THE ESTIMATED TIME OF ARRIVAL IS OFF AS MUCH AS 50 PERCENT. What'* the problem?

Estimated time to arrival is calculated based on driving the rest of the trip at the posted speed limits, not the speed you are averaging. If you are driving over the posted speed limit you will beat the estimated time of arrival every time.

Probably doesn't help, but that'* all I could think of.

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.