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speedometers are those kind of speed are WAY off, so don't rely on that
just go with the manufacturer speed limiter, and when your reach fuel cutoffs, that's exactly the speed you were going
+1, the faster you go the more off it is (they generally are off by a number of percentage points, and the norm is for the spedometer to be conservative). You can calculate your true speed by using a portable GPS. I know on my motorcycle they purposefully advance the spedometer by quite a bit.
The GPS doesn't lie though and is generally very accurate at determining your true speed.