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A great way to verify this would be with an RPM app like this one for iOS or this one for Android. Ultimately, a stopwatch and counting rotations is the best way to measure speed, but it sounds like you're dealing with some wow & flutter, so watching the strobes on the side and using that app can provide a bit more detail. If it turns out the PLX is not spinning smooth and accurately, don't hesitate to take it back to the dealer for an exchange. Let them know the issue and they'll swap it for you.