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veLLo-
You may even need to inflate below recommendation to get the best range of grip, speed and comfort.
MTB tyres usually recommend 40psi (ish), but even then, dropping to 20psi make a massive difference in grip allowing the tyres to spread more on the ground, ditto to cyclocross tyres that people often run at 25-35psi, way below the recommendation of those tyres just so it'll work in the mud.
edscoble
Probably because the knobbie on the side don't offer much grip compare to slick tyres, plus if you rode them above idea pressure, it's very slippery.