[TestDrive]

The picture showing "formula for aerodynamic horsepower" was lost when this got moved to a new thread.
Here's one from Wikipedia.

Horsepower required to overcome aerodynamic drag -

is the force of drag,
is the density of the fluid (Note that for the Earth's atmosphere, the density can be found using the barometric formula. It is 1.293 kg/m3 at 0 °C and 1 atmosphere.),
is the speed of the object relative to the fluid,
is the reference area,
is the drag coefficient (a dimensionless parameter, e.g. 0.25 to 0.45 for a car), and
is the unit vector indicating the direction of the velocity (the negative sign indicating the drag is opposite to that of velocity).