You will find that almost all GM cars call for 30 psi front and rear regardless of tire size, even if you change sizes. I have noticed some center wear so I typically will run about 28 psi. As for the most fuel efficent, well max psi will give the most fuel efficent, but worst ride quality. Nitrogen is a relatively new concept, but keeps the tire at a lower operating temp helping to wear a little bit longer, you will lose less of it due to temp changes, but it is a PITA trying to find a place that has it if you need to adjust your pressures. If someone dosent have it you need to let all your air out of the tires before adding the "regular" air.