Can you substitute P255 75 R17 P255 75 R15?

No, you cannot substitute P255/75 R17 tires with P255/75 R15 tires. The numbers indicate the tire's width, aspect ratio, and diameter; in this case, the first number (17 or 15) represents the rim diameter in inches. Since the rims are different sizes, a P255/75 R17 tire will not fit on a vehicle designed for P255/75 R15 tires.