i'm sure this has been discussed before but i couldn't find it on any recent posts. I have 205/40ZR/17 Nitto 555 Extremes. In one place on the tire it says "never exceed 40psi" and on another place on the tire it says "never exceed 50psi". It almost seems like my tires are a little bit flat, but i put them all at around 37 or 38psi. I was wondering what should I put the psi at?