Mine is definately a little slow due to taller tires. Speedo at 60, GPS reads 62/63. At low speeds, they are closer together.

 

Instantaneously, there is a possibility of that. Over a 20 or 30 second average with clear reception, it should be WAY better than 0.1%. Over a 5 minute average, it would be within 0.01%. This assumes a speed, IN A STRAIGHT LINE, of at least 30 mph. Time difference is substantially, as far as we can measure, perfect. Distance traveled could be off, in clear reception, perhaps, PERHAPS, 100 ft.