First number is the tire section width in Millimeters, second number is tire section height as percent of section width.
So a 285 is about 30mm (1.18") wider than a 255 tire ( 285mm-255 mm = 30mm ) and due to the added width may cause some rubbing at full steering lock or full suspension travel on stock wheels.
Overall diameter on the 285 is also slightly larger
255/75R17 --> 255 / 100 X 75 X 2 / 25.4 + 17 = 32.06" Diameter
285/70R17 --> 285 / 100 X 70 X 2 / 25.4 + 17 = 32.71" Diameter
Note numbers are only approximate and actual numbers vary in either direction from the indicated tire sizes depending on tire brand, tire type and wheel width the tire is installed on.
Very Nice Disclaimer at the end! I like it.
Paved Roads - Another Fine Example of Unnecessary Government Waste!
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