[math lesson]
the first number, 315, is the tire width in milimeters (mm). To convert mm to inches, divide by 25.4. So 315 / 25.4 = 12.4".
The second number, 70, is the sidewall aspect ratio as a percentage of the width, so the sidewall is 70% as tall as the tread is wide. So, 12.4" x 70% = 8.7".
Since the overall height includes "2" sidewalls (tire is a donut shape), multiply the 8.7" by 2, then add it to the rim size (the last number) to get the overall tire height
So, 8.7" x 2 = 17.4" ; 17.4" + 16" = 33.4".
[/math lesson]