Instant speed is a funny idea. You say it the same way you talk about constant speed or average speed, but you don’t mean the same thing. It’s kind of like saying, “If at exactly 2.15823 seconds after launch the shuttle stopped accelerating, it would continue moving at a constant speed of 183.8964 miles per hour.

It means that if you were to find the shuttle’s average speed from 2.158225 seconds to 2.158235 seconds, and round to four decimal places, you would get 183.8964 miles per hour.