For off roading with my new ML500, I would like to lower the tire air pressure for two different kinds of driving: slow (1-5 mph) rock crawling, sand and very rough road travel; and 40 mph travel on warshboard gravel and dirt roads. In my Jeep wrangler which is being replaced by the ML, I lower the pressure in the 9.5X30 B.F. Goodrich tires to 18 psi for these conditions. This has worked well for me. Does anyone know what a safe pressure for the 275/55R17 tires on the ML?

the only time it makes sense to lower tire pressure for offraoding is when you are in sand .... rough conditions and rockcrawling it is a bad idea because the wheel is more likely to be damaged .... but, in the sand in an ML I have seen as low 20 psi by a few people I know

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Thanks for the info. I will probably try 20 psi. I dissagree about only on sand. I find that I get a better grip on rocks and am less susceptable to sidewall damage because the tire will give more. You do, however loose an inch or so of ground clearance. Wheel damage might be a problem. On washboard roads the softer tire gives a smoother ride and it seems easier to find the speed that smoothes out the washboard.