I think we're just talking about tyres that are designed for on and off road use but even then you have three basic categories: Road tyres like TKCs which have a rigid sidewall so when the tyre pressure is dropped from the recommended the tyre only deforms by a relatively small amount allowing the tyre to become flatter and give better traction in sand and mud but only to a point. If you drop the pressure too much the tyre will just overheat and the sidewall will break up. Enduro tyres like Michelin Comps are more flexible so you can run then down to less than .5 bar if conditions need it but they are horrible on a paved road at that pressure and wear very quickly. Also, obviously, need rimlocks. Trials tyres are very flexible and are designed to be run almost flat but again feel bad on paved roads at low pressures and still tend to overheat and wear very rapidly. As with everything dual purpose, tyres are a compromise. True road tyres grip the road in a completely different way to the way in which a full motocross tyre finds traction on muddy terrain.
I use 1.5 bar (23psi) front and rear in the TKCs on my GS (no rimlocks) on all surfaces but wouldn't go anywhere near some of the mud I go through on my KTM Exc fitted with Michelin comps 1 bar in the front and .6/.7 bar at the rear.
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