Here's the break down.
285 is the width of the tire in MM
70 is a percentage of the width and represents 1/2 of the tire sidwall
17 is the diameter of the opening of the whole for the wheel in inches.
25.4mm to every inch.
So heres the math;
285mm x 70% = 199.5mm
199.5mm x 2 = 399mm
399mm / 25.4 = 15.71"
15.71" + 17" = 32.71" (diameter of the tire)
285mm / 25.4 = 11.22" (width of the tire)
17" (diameter of the wheel)
285/70 17 = 32.71/11.22 17
But be aware, the actual measurements of a tires diameter and width are almost always going to be slightly different and very from brand to brand.
Making these numbers up, but as example, a 285/70 17 BFG KM2 may be a 32.3/11.5 17 where as a 285/70 17 Mickey Thompson Baja Claw could be 33.1/11 17, if you measured their actual dimensions.
Hope this helps. Good luck.