The reasson to air down your tyres is not just for grip. If the tyres are at normal pressure and you hit a small sharp stone at speed, it will ripp the inside of the ply on the inside of the tyre and over time will cause the tyre to blow out when you drive at speed on the free way. Any time you get on a dirt road I would say deflate to 1.5 bar, eaven if you have road tyres on the Jeep. As sson as I get on a dirt road I deflate to 1 bar, but I have 33" BF A/T tyres on my Commander.