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When I researched the explorer recall w firestone tires I found that Goodyear endorsed having 28 psi was optimal for those trucks. My suspension is the same as an explorer. After testing tire pressures I found the the placard's spec of 30 psi was too high, and 28 in the front got optimal wear from the tires. But the GOV'T stepped in and said to change explorers to 30 pis and Ford complied. Currently all the rangers I have dealt with running 30 psi have premature center tread wear for front tires.
With relation to Diameter, from one point of the tire to the opposite side (via center tread) the differences between loaded measurements from top to bottom are different from leading and folowing measurements, therefor the top of the tire to the contact at the road is the best applicable measurement since that is where contact is made) the circumference is altered at center tread by the diameter x pie unloaded. Loaded makes the vehicles front and back measurements for the tire's diameter at 9 o'clock and 3 o'clock measurements different from the 12 o'clock and 6 o'clock measurements.
My ride height change was measured at the position directly above the peak of the tire (12 o'clock) the fender and bedside's wheel lip at the outer most edge and lowest point was the position from witch my measurements were derived. Choosing another location on the vehicle would show similar results but the clearance fro the tires was most important and is the reason mini truckers like to tuck the largest rims possible.
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