I always just ran at the mfg recommended pressure and never did a chalk test on set of tires until today. Well, I'm doubting the results a little bit.

I have 275/55-18 Michelin Cross Terrains on my X. I have been running them between 30-32psi and they have a "Max tire pressure" of 35psi.

The results of the chalk test tell me that my optimal pressure is 40psi in front and 38 psi in the rear. At those pressures I have nice even wear across the entire treadwidth. I ran it a few times.....pumping up the tires like balloons, chalking a thick line across each tire, driving 100 feet or so, then inspecting for chalk wear. If I lower the pressure to 32psi or below then I'm burning more chalk off the eges than in the middle.

I obviously can't run 'em above the max pressure or I'm pushing the limits of the tire. Is it safe to run 'em at 40psi? Do I just set em at the max of 35psi and fuhgeddaboutit?

Hey Man, I had a set of P265/70/R16 Cross Terrains, and I was running them at over 40psi, probably near 50psi for quite some time (6 months), and I decided to lower them a little bit to soften up the ride. The tires seem fine and there are no unusual bulges or anything that would look abnormal. You can probably put a little more air in the tires.

Although I don't normally try to follow mfg's instructions.........OKAY I'll leave 'em at the optimal treadwear pressure. They are an all season tire, so they are a pretty decent ride even at the higher pressure. The 275-55-18 handles just as well as my 275-60-16 Scorpion Zero's did.....except the Xterrains give me a little squeal now and then when I mat it. :D

The chalk test won't be very accurate with more than 5000 miles on the tires.

Ahhhhhh, he's everywhere, he's everywhere.....but I appreciate the boundless knowledge! :D

Good tip. The tires have less than 2kmi on 'em and there is no visible or measurable wear from stock with digital calipers. I'm guessin that with the 9" wide wheel and all-season sidewalls that most of the pressure is directed to the outer tread blocks. No sweat, to my complete surprise it rides well with 40psi/f and 38psi/r. IAS shocks 4EVA!

LOL.

If there's visible wear then the test isn't very accurate, but if you find them good with that much air, go for it.

Mine is on it's second set of XTerrains.
Ran 40 psi all around on both sets with no problems.

Well, there was that dissagreement between one of the first ones and a 1/4 bolt, but I don't think the pressure had anything to do with that.:p

I ran about 32-34 in mine, as low as 12 offroad, about 40 when towing.

Dowwwwp! Sorry Alec, what I meant to say was there is no visible or measurable wear. Trying to sneak in threads while at work and typing too fast again! :D The test should be accurate, and I'm happy at 40psi for THIS application. Lata