The SI unit for speed is meters per second (m/s).

Here's why:

* SI (International System of Units): A standardized system of units used globally in science, technology, and commerce.

* Miles: A unit of distance in the Imperial system (used in the US and some other countries).

* Meters: A unit of distance in the SI system.

* Hours: A unit of time in the Imperial system.

* Seconds: A unit of time in the SI system.

To convert mph to m/s, you'd use a conversion factor: 1 mph ≈ 0.44704 m/s.