Here we understand the speed we read on the airspeed indicator. Unfortunately, it is not the case that we really move at this speed. The airspeed indicator shows too little with increasing altitude!
per 1000 ft altitude approx. 2 % too little is displayed

If you now want to know how fast you really fly, there is of course again a small formula: first the calculation of how much in percent too little is displayed:
height in feet / 1000 * 2%, now we know how much % is displayed too little
e.g. FL 300 with an IAS of 260 Kts. 30000/1000 = 30 * 2% = 60 % are displayed too little. This corresponds to 156 knots! (26 *6=156)
TAS = IAS (260 kts) + 156 kts = 416 kts true speed at an altitude of 30.000 feet (FL 300)
This reads much worse than it actually is, especially since the TAS is also displayed in the modern panels, so you can save yourself the computing work :-)

This speed only comes into play when we are not flying in calm conditions.
As we all know, this is unfortunately not too often the case, unless I am a "fair-weather-flusi-pilot" who doesn't think much of real weather.
He can save himself the trouble of reading on and immediately ascend into the cloudless, windless, beautiful blue sky :-)

Calm

When the wind is calm, I fly Speed of 250 kts TAS,
then I also move above ground at a speed of 250 kts GS

Tailwind

I fly with a tail wind of 30 kts and a TAS of 250 kts,
then I move above ground at a speed of 280 kts GS

Headwind

I fly with a head wind of 30 kts and a TAS of 250 kts,
then I move above ground at a speed of 220 kts GS

 

The TAS is never affected by the wind, but only the GS and thus the time in which the flight destination is reached!

The calculation of the GS shown here is intended for us PC pilots up to a crosswind of approx. 15° are valid and is quite sufficient for flying on the PC.