This speed is quite stable with time since Earth takes 23h56m4s to make a full rotation but is varying by latitude and altitude.

For instance, speed is 707 mph for Seattle and 940 mph for Miami.

Have in mind that Earth travels thrue space within 67,108 mph, around the Sun.

Formula?

2.π.R.cos(ϕ)/t
R = radius (20,903,000 feets)
ϕ = latitude
t = 86,464 seconds

Physicists and mathematicians will be quite astonished to see us calculate linear speed to evaluate this rotational speed (expressed in radians per second ).

Note that we are talking about linear speed, which is expressed in mph and more intuitive for most people. (Astro)physicists will rather use angular velocity, expressed in radians per seconds.