Why do FM's get slower with altitude instead of faster, like performance data claims?

[konate]

In Bismarck's 2 latest videos, there is a graph of the speeds of the Yak-1s at various altitudes and they are fastest at SL and slowest at altitude, the opposite of what the specifications provided in-game claims.  

 

Why is that?

 

0:14

 

 

Yak1b speeds.bmp

[216th_Jordan]

Its the indicated airspeed your instruments measure. Indicated airspeed differs greatly from the true airspeed of your plane the higher you get:) Its good to read yourself into it a little:

 

https://en.m.wikipedia.org/wiki/Indicated_airspeed

[konate]

Ah.....

 

Simple enough.  

 

Thanks, I missed that.

[Finkeren]

Simple: The graph shows indicated air speed (IAS) not true air speed (TAS)