Another GPS Idea for Saving Fuel

I know this is probably a far-fetched idea, but it's something I thought of that would also save fuel using GPS technology. If our automobiles had a way to send our vehicles location information while we're driving up to a system that records x, y, and z coordinates, it would only take a few weeks for our driving efforts to automatically and fully map the entire highway network in the USA. After the recording and mapping of all traveled highways x, y, and z coordinates (remember the z coordinate is the elevation), that information could then be used to help control our automobiles engines behavior. With a mapped system of coordinates and locations, the GPS system would be intelligent enough for our automobiles to "know" if a hill or stop was coming and allow for reduced number of cylinders and/or a rev limit on the engine that would help reduce speeds in anticipation of a stop or of a hill to climb. The system would help aid the automobile in saving fuel as well as avoiding accidents. Just think, your car would stop accelerating as you come to a stop light (you would still have control but your car would not allow you to accelerate towards a stop light. And, your car would start to decelerate longer than what most humans do as they wait to the last second to touch the brakes for a stop light). Increased safety and fuel savings, why not?