Based on my "instincts" that have grown over years using various simulations I suspected something to be not so kosher with actual speed indications. I made a few measurements to verify my assumptions.

Measurement 1:

Covering 1 min latitude exactly North-South direction with the old Tatra.

- Time used 1 min 10 sec for 1 NM = 1852 m -> average speed ~95 km/h

- Speed indication on the Tatra's Speedometer -> >90 km/h which seems to be correct

- Speed indication on HUD average 27 KTS -> ~50 km/h which is far too slow

Measurement2:

Covering 1 min climb with the Cessna at indicated ROD of 500 FPM.

- Altitude gain approx. 250 - 270 ft which is only half of what is should be

- speed indication HUD vs. IAS indicator is coincindent which means that the Cessna is flying far too fast (reaching some 105 KIAS on straight flight). Furthermore the speed indication is always coincident with the IAS indicator of the A/C. I assume that the HUD deals with actual angular (ground) speed. Therefore the IAS of the A/C should become smaller with height.

Edit:

Measurement 3:

Covering 10 min. latitude inflight with the Cessna in 5:50 min

- groundspeed of approx. 102 KTS which is correct according to the HUD's indication

Measurement :

Flying at 1000 ft indicated altitude (Cessna's altitude indicator)

- ~3250 ft altitude on the HUD

It seems that the speed indications/calculations are presently a bit at loss. The only reliable speed indication seems to exist on the old Tatra

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