285 = the width in mm. 25.4mm per inch mean that 285/25.4= 11.22" wide.
245 mm wide = 9.64" wide
The middle number (70 in a 285/70/17) is called the aspect ratio. In laymens terms, this tells you how tall the sidewall is as a percentage of the width.
70 = 70%
70% of 285 is (about) 200. So the sidewall height is 200mm (or 200/25.4 = 7.8")
Then to figure out the diameter of the tire, you just add up sidewall+wheel size+sidewall = 7.8"+17"+7.8" = 32.6"
In conclusion, the standard translation for a 285/70/17 is 32.6x11.22
.... want to know a really EASY shortcut?