710/70R38

710 is the width in millimeters. 

70 is the aspect ratio.  This is the height from the ground to the bottom flange of the rim divided by the width, so in this case that figures to 497mm.  

38 is the diameter of the rim in inches.  Why inches?   Who knows.

So, doing the math, the radius of this tire is (1/2x38inchsx25.4mm per inch)+(.7x710mm) or 979.6 mm 

The 650/65R42 calculates as 905.1 mm

The 620/70R42 calculates as 967.4 mm

The 620/70R42 and the 710/70R38 are close enough that they are probably considered to be the same height from a design standpoint.  The 650/65R42 is in the next shorter family.