Do you want knots, mph or kmh?
Set up a course that you have measured a specific length, time it, and wala, there's your speed.
One you have determined that speed you can calculate into airspeed.Many factors come into play.
The higher an aircraft goes the faster it flies at a given power setting, because there is less drag. And it has nothing to do with ground speed, but with true, as opposed to indicated, airspeed. But there is less air density at higher altitudes, so the higher you go the less power is available.
True airspeed is the speed of your aircraft relative to the air it's flying through. ... In fact, for every thousand feet above sea level, true airspeed is about 2% higher than indicated airspeed. So at 10,000 feet, true airspeed is roughly 20% faster than what you read off your airspeed indicator.
In simple aircraft, without an air data computer or Machmeter, true airspeed can be calculated as a function of calibrated airspeed and local air density (or static air temperature and pressure altitude, which determine density).