As cnckart explained, the quick, easy way to convert a metric measured tire size (e.g., 285/70/17) into something equivalent to 33 x 12.5 (e.g., tire diameter and width in inches) is the following:
The first number on the metric sizing is the width of the tire in millimeters. Take it and divide by 25.4 to convert to inches. So 285/25.4 = 11.2 inches. That's the tire's width.
Multiplying the width by the second number in the metric sizing (which is the sidewall ratio and is expressed as a percentage) gives you the distance between the outer edge of the tire and the rim, or the measurement of the sidewall. So 285 x 70% = 199.5 millimeters. Again, to convert to inches divide by 25.4, so 199.5/25.4 = 7.85 inches. Now to determine the overall diameter of the tire, you must take the sidewall measurement, add in the rim size (which is the third number in the metric sizing and is actually measured in inches, so in this case it is 17 inches), and then add in a second sidewall as the rim has sidewall surrounding it (e.g., the diameter of the tire itself would consist of the measurement of two sidewalls and the diameter of the rim). So you have 7.85 inches (sidewall) + 17 inches (rim) + 7.85 inches (sidewall) = 32.7 inches.
So 285/70/17 = ~32.7 inches in diameter x 11.2 inches in width. The rim size is 17 inches.
Hope this helps.