That your mpg went down from higher psi is the opposite of what's supposed to happen. Higher psi, to a certain degree, reduces rolling resistance and will also make you ride on less of the tread, depending on wheel width. A 12.5" wide tire on an 8" wheel at 60 psi is going to go bald in the center and have deeper tread on the outsides.
To find optimum tire wear, run chalk across the tread and adjust the psi to until it all scrubs off. I would start at max psi, then reduce psi until the chalk mark is scrubbed off. However, this only works on NEW tires. Older tires will have a flat surface to the road, but tread depth across the tire can vary. I run 315-75-16s on a 10 inch wheel and 40~45 psi is my optimum psi for best tread wear.