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That sounds like a conspiracy theory. They are used and relied upon for so much in civilian world, including aviation, shipping trucking etc, that they do not muck about with them. In fact if you pay them some (a lot of) money they will even unlock to allow pinpoint accuracy within a few millimetres. That is used for all sorts of things nowadays.ALL GPS satellites are military run, therefore they can change things as and when they want ( and they do ) ever wondered why you have a great signal one second and then the next none!!!! - therefore you cannot rely on the speed displayed to be accurate NOT EVEN to +/- 0.5mph at worst.
And anyway, as others have said there are plenty of alternatives to the US military system now. And if you were correct my sat nav would be forever losing where it was, but it doesn't most of the time. In fact it only happens in cities with lots of high rises, because of the canyon effect or when under tree cover in the summer. And if the signal is lost it is not be able to display the mph anyway.
As I said they are much more accurate than the speedo for the reasons I have given.