How do we measure or think of speed in the united states?

I'm not sure what you mean by "think of speed", but the way we measure it is by plugging the numbers into this formula: distance divided by time. If you are trying to find an average speed, then your formula would be: total distance divided by total time. I hope this helped!:)

whichever one it is, it's definitely not one that you included on the list you provided.

if they have a finite resultant