ops sorry, I changed my mind while you wrote your post AndyJWest and I deleted last post, I'll repost here;

http://www.flightsimaviation.com/rul...speed_TAS.html

In this link it say 2% in #3, but after I checked #5 that seem more accurate, it shows a significant difference.
I tested with a Mustang Mark III 100% power, at 27500 feet in IL2 it show 620km/h TAS (344 knots), and 380km/h IAS (211 knots)
#5 27,5*3 (82,5 )+ 211 + 7 = 300.5 knots TAS ,difference is 43.5Knots.
It got me thinking, wouldn't this make it more fun to fly at higher altitudes, if this is more correct, it would sure improve flight, because this would give a much higher IAS, and that would reduce speedstall problems at higher altitudes.

I hadn't realised you'd deleted your last post, Buzpilot. I've edited mine to avoid confusion.

The percentage figure is only an approximation - the relationship isn't totally linear, and in any case only holds under standard atmospheric conditions. None of the stock maps reproduce this, though I believe that Smolensk is the closest.

There is more to high altitude performance than just the IAS/TAS relationship in any case, and I don't think there is a systematic error in IL-2 due to any miscalculation.

I'm not sure, if the formula in example #5 is correct( It say; Normally accurate to within 5 Knots.), it indikate just that.
And many planes in real life ww2 had their best combat altitudes above 20 000 feet, but nobody makes missions at those altitudes in IL2, not just because it takes a while to get there, imho .

Edit; I don't think this is a error, but maybe too simplified, and after comparing lots of planes in il2compare, I was amazed of the amount of data in flightmodels, and it probably would get even better with more advanced IAS calculation.