@easytiger
One thing I forgot to add: the three readings are 551, 571, 576km/h.
If you would be right and there would be an 50mph error then maybe 576km/h (355mph) is correct and the GPS should show 405mph. Or the other way around. The plane flies 355mph and the GPS shows only 300mph. If there is an error it could go up and down. The lowest speed is just 4% away from the highest. So in your opinion the speed should be somewhere between 300mph and 400mph. But the range (551-576km/h)is very small. If there is an error then why will it always go up? And if there is an error why is it always in the same range? If there would be an error the GPS would show something like 301mph, 347mph, 417mph. Why goes an error always in the same direction? Why would have these errors all the same diffence? If there is no error the GPS shows similar numbers. And this is the case here.
Niels