2. The different OTR tire familes

100 Series ( Narrow Base )
The H/S aspect ratio is approximately equal to 1.00
The section width, given in inches is a whole number
Example: 18.00R25

80 Series ( Wide Base )
The H/S aspect ratio is approximately equal to 0.80
The section width, given in inches is a whole number followed by a fraction
Example: 20.5R25= 525/80R25

65 Series (Low Profile Tires )
The H/S aspect ratio is appoximately equal to 0.65
The section width, given in millimeters followed by the number 65
Examplt: 750/65R25 
3. Comparison of tread depth