Having done some rallyx tests recently, I noticed quickly that driving a car with high pressure (ie : > 2 bars), make you much faster on the 'dirt' parts of the track, while running with low pressure allows a better handling (means faster) on the tarmac part...I've always heard that low pressure are needed on sand and soft ground to give a better traction...but in LFS that does not seem to work (test the little track (FE6) if you did not notice this already)... Am I right on the theory??

With sand there is no hard surface underneath for the tyre to grip so you need low pressures to stop the car digging in. With dirt and snow there is usually a hard surface underneath so you can run higher pressures (and in the case of snow very narrow tyres) to dig in and grip the hard stuf. Thats my understanding anyway.

yea, as long as there is something hard underneath the surface, you can run a higher tire pressure.

i usually run around 25psi in the front to get the bite into the corners and around 30 psi in the rear help increase oversteer..but that's just my style

tarmac is by far harder, and with high pressure that sux
So, the problem should be at Fern Bay only (I always thought that is sand...) and not at Blackwood...

id say it has alot to do with the suspension u can have almost same effect when u have soft suspension and hard tires or reversed.
after all of course softer tires (should) have some more grip but it isnt a big difference.
and softer tires give u more grip for accleration while more pressure gives u some more grip in turns (the wheels dig into the sand / dirt and get that way grip) so i think its right how it is.

with high tyre persure it allows to expand the tyre and gives the tread more contact with the dirt and allowing more speed to be given but with the disadvantage of less grip