It's the other way around: the speedo can overestimate your speed but not underestimate it. If you follow the limits with an overestimating speedo, you drive under the limit. With an underestimating speedo, you end up over.
Anecdotally, when I pass those roadside speed alert signs, the speed they show and the speed on my speedometer is rarely more than +/- 1 mph. I think modern speedometers are pretty accurate, as long as the OE tire size is used.
That my experience too, the speedometer, speed my phone thinks I'm going, and static radar signs all more or less agree. Plus the only times I've been nailed for speeding, I was speeding, not "just kissing" the limit. Point being I don't think +/-1mph really matters in practice 99%+ of the time, it's usually getting tagged when being overconfident in the passing lane or something like that.
