Why don´t these figures match up?

[IvanK]

I suggest you do everything at sea level. 500m is to high it introduces a real difference between Indicated Airspeed (IAS) and True Air Speed (TAS). Doing the test at sea level pretty much eliminates this as IAS is very close to TAS.

I don't see any correction in your results for TAS. 500m Altitude on a Standard day would result in around 6Kmh diff between IAS and TAS.

I will have a look at 3min test like yours but at sea level only. Personally I think 3 mins is too short a time period and magnifies any errors. You also need to always fly the test both ways to ensure cancellation of any wind effects. In addition I think you would be better off doing ALL your tests between to fixed markers a known distance apart rather than flying a time and seeing where you end up. There are issues transferring your perceived position to the map then measuring.

Nothing to stop you precisely placing Ships at 10000m intervals using the Grid on the FMB map and using this as a test track. Timing between features a known distance apart is the way to go imo.