At what altitude are true airspeed and indicated airspeed most different?

True airspeed (TAS) and indicated airspeed (IAS) differ due to atmospheric conditions, particularly air density. At sea level, the air is densest, which means that indicated airspeed closely reflects true airspeed. As an aircraft climbs to higher altitudes, the air density decreases, leading to a greater difference between TAS and IAS. In this context, indicated airspeed is affected by the reduced pressure at higher altitudes, which causes it to under-represent the actual true airspeed of the aircraft.

Thus, at sea level, the two speeds are most aligned because the effects of air density are minimal. Conversely, as altitude increases, indicated airspeed does not rise as quickly as true airspeed due to the thinning air, resulting in a greater disparity. This is why the altitude at which true airspeed and indicated airspeed are most different is not at sea level, but rather at higher altitudes.