Is average speed measured in miles per second?

Answer 1

No. It is usually measured in miles per hour.

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(Any unit of length or distance) divided by (any unit of time)

is a perfectly good unit of speed. You simply pick a unit that works well for the

particular case you're working on. A good unit is one that people understand, and

produces a number that's easy to write down, remember, and tell other people about.

The same considerations are applicable whether you're talking about top speed,

lowest speed, instantaneous speed, or average speed. (Also magnitude of velocity,

which is really just speed.)