So i'm looking at getting a smittybuilt air system so I can air down on trails. I am running procomp 15x8 and 35x12.5r15 KM2s. Sooooooo I was wondering what a safe airdown psi would be. The terrain here goes from Sloppy mud, Sandy mud, High water, Rocks, Lava rocks, sand. So just about everything lol. Also any real life experiences would be great along with recommendations for other peoples set ups. THANKS

 

Trial and error is really the only way for you to figure this out. You'll soon find that the optimal PSI for you is a compromise between ride quality, traction, and the time required to both air down and pump back up. If you wheel often enough, you'll have plenty of opportunities to experiment.

Try 15-20 PSI the first time and adjust from there. Too low and the tire will slip on the rim and throw the wheel/tire combo out of balance. Way too low and you'll pop a bead and the tire will come off the rim.

 

I run a 15" procomp wheel with 35" km2s and wheel at about 12 or 13 psi. Seems to work in just about any terrain I am in and haven't ever worried about popping a bead.

 

I have 35inch trail grapplers on 17 inch wheels and always air down to 15psi works out great.

 

What will you be doing off road? You'll want really low for crawling. But, for overlanding/expedition/headed to the hunting camp travel, keep the pressure about 18-20. I have two ripped open sidewalls on two different brands of tires as justification for that suggestion. And now carry two spares.

 

Recommendation-I usually air down to about 12-14psi max in my 315/70R17 Goodyear Duratracs.
Experience-I aired down my first time in my Rubi and it took 2 month of driving till one day the light finally turned out (after airing back up of course) about 2 weeks after it turned off I aired down again and it's been on ever since June 2013. I even drove around for 2 weeks with 45psi just to make sure they had enough air in them.... Light still on.

Sent from my obama monitoring device

 

Its really about preference and tires. I take my MTR's down to 8 on the rocks. I say start somewhere around 18 and play with it, see what feels rite.

 

I have 37"km2 and usually air down to 15 to 20 psi on the trails. I have a Viair 450 compressor and takes me about 45sec per tire to air up.

 

10-12 psi for a 12.5" wide load range C tire on an 8" wide wheel. Higher than that if you had a 10" wide wheel. Lower than that if you were running a load range D or E tire. Higher than that if you have a heavy rig.

When I lead a group trail through mud, snow, sand, or rocks, anyone with more than about 12 psi is asked to stay in the back of the group so the rest of us don't have to wait for them to make multiple attempts at obstacles the rest of us drove right through.

The exceptions would be rigs that are very heavy or running wider wheels, or tires that don't have a 10" tall sidewall like yours.

If the rocks are jagged and sharp, or the speed is more than 3-10 mph, then we typically run more psi.

It is more frustrating to have to wait for people to make multiple attempts at an obstacle solely because they haven't aired down enough than it it is to help someone reseat a bead on a very rare occasion.

Airing down elongates your tire tread pattern contact on the ground and to allow the tire to conform/wrap around obstacles. I haven't measured, but the difference in traction for a 35x12.50R15 from 20 psi to 10 psi would be substantial.

 

X2 on this. 15x8 and 35's, I ran no more than 15 psi for any wheeling (never done high speed stuff so don't know about that). 10-14 is a good range