I'm sure it is. Irrefutable testing method.
In a school zone, it better be pretty damned close. If you see a discrepancy at 25mph, there's a problem. Conversely, if you're going fast enough to see that your speedometer is 5% off, you should probably be arrested.
My point is this - your speedometer might look pretty accurate at 25, 35, 45mph - even with as much as a 5% error. At 150mph, it's another story. A 5% difference is much bigger at those speeds, and makes these 'I went 172mph and I can prove it with another car's speedometer' arguments worthless.
My SVX's speedometer is pretty damned close - definitely the most accurate I've checked against GPS. But I wouldn't dare say that at 150mph it's 'DEAD ON.' I think at 100mph it might be a couple mph off. That was years ago that I checked it like that - who knows these days.
Just to be clear, I'm not calling you a liar. It is possible, just by chance, that your speedometer is 'perfect.' I'm just saying that automobile speedometers are not the most trustworthy, accurate measuring devices. That's all.