On my 07 Civic EX I'm running oem spec 205/55/r16 and the door jam says to fill them to 32psi. My nitto tires say 35psi and i have them filled to 40psi.

What will this do?

better or worsen performance and mpg?

 

higher psi will net you more mpg, but will worsen comfort and potentially performance.

also, high psi can potentially cause tire bursts from
impact (potholes) as the tire has no room/give to absorb.

depending on tires i use, i am usually between 34-38psi cold.

 

Yeah if you're not using OEM tires, forget what the door jam tells you... read the tire and go with that, if the tire reads 35max psi, i would run between 30-33, but thats me

 

i run it right at 40psi get great mpg