[f5fstop]

I will have to guess and say most of the time...no. The AVIC I installed looks at the VSS for correction, especially when in heavy tree coverage, long tunnels, cities, etc., but will operate without the VSS. However, the AVIC's main input is the actual GPS antenna.
So, not saying the factory is the same, but it is made similar to the aftermarket units. Besides, GM does not make the unit, and aftermarket supplier does.
So, if running in heavy coverage the unit might be incorrect, but when coming out of the heavy coverage, the GPS should correct itself, and the correction would be slight.
Again, just my opinion on how it should work.

More information from someone who knows this system pretty well:

The vehicle speed signal is used for greater accuracy for determining the position of the vehicle when GPS signals are unavailable.