Planetary Speed Limit

Tina Shaycort of Melissa, Texas, writes:

Marilyn: When a person travels by car at, say, 50 mph, this speed is relative to the road. You stated that New York City spins at about 775 mph. (September 13, 2009) What is this high speed relative to?

Marilyn responds:

The speed is relative to a point at New York City's latitude, which circles the planet like an imaginary road.