The 285 is tire width in mm from sidewall straight across to sidewalls. The 70 is the sidewall height % of the 285mm. The last one as you know is rim diameter.
So 70 is about 200mm tall(8"). That's your sidewall height. A 285/70/17 tire is approx 33" tall overall.
So 70 is about 200mm tall(8"). That's your sidewall height. A 285/70/17 tire is approx 33" tall overall.