Does everyone else seem to get black carbon build-up on the exhaust tips too often? It seems I remember someone else mentioning it, but don't know what they found out. I have to take a rag and wipe around the rim of the tips after every drive to remove the residue. Seems like only cars with a bad tune-up and a "rich" air-fuel mixture would do that so regularly. Maybe that is why the mileage is worse early on and improves around 2000+ miles as others have said. Does GM have the computer programmed to run a richer fuel mixture for the first couple thousand miles? Just curious.