[IvanK]

Here are the test results using Spit IIA. Again feel free to pick this apart both Maths wise,method wise and the conclusion.

Firstly I checked the file properties and ensured that all "Flow" and "wind" values are deleted. However they always return when you next enter the mission.

METHOD
Same as before however 261MPH (420Kmh) was a bit too fast to achieve with the current CEM/FM so elected to use 240MPH on the ASI this also gave a simple reference to fly using Wonder woman display mode. Again East bound times were shorter than West bound times. Implying that some latent wind is still present on the map. 2 runs each way were flown. IAS maintained to within +-1mph each run. I then averaged as per JTD's suggestion by average Eastbound values then average westbound values then a combined average.


RESULTS:
East Bound Run 1: 6min 14secs
East Bound Run 2 : 6min 12secs
Avg of both East bound run times : 6min 13secs

West Bound Run 1: 6min 36secs
West Bound Run 2: 6min 33secs
Avg of both West Bound time: 6min 34secs

Overall Avg time for 42.59Km: 6min 23 Secs.

Converting MPH to KMH 240MPH = 386.3KMH

Time to fly 42.59Km at 386.3KMH IRL 6min 36secs
I flew 42.59Km in an average time of 6min 23secs
42.59Km in 6min 23secs works out at speed of 400.53Kmh
So speed error is 13.23Kmh (8.2MPH)or about 3.6%

CONCLUSION:
Again considering the potential errors in the test method same as BF109 case I think its close enough. Spitfire ASI is accurate. Its worth noting that even IRL you will get some Instrument errors. All Flight test reports will have some correction table.

Notes:
No matter if you delete all "Flow" or "Wind" some latent wind is always present on the map.

Maybe significant for the level bombers and explain some of the strange IAS/TAS sight setting issues and workarounds that are being employed to get bombs on target.