I
have included an ancient chart that is a way to easily calculate true airspeed.
For all practical purposes the
IAS is approximately equal to CAS at all air
speeds
And CAS, at the higher the altitude, the more the
pilot must subtract to attain the equivalent air speed (EAS).
TAS is EAS multiplied by SMOE 
.
But for my testing purposes I am not reporting
airspeeds -- only difference and direction of change. I want to go faster, and
cut time off the clock for given routes and trips. And save gas
Charlie K.