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1.1   Tire Size

The size of each tire is indicated by nominal width and rim diameter in inches and mm.

Radial Structure is indicated by the letter “R”. For some tire the aspect ratio is indicated by percentage.

Example

Radial Tire: 23.5R25 , 18.00R33

Bias Tire:  23.5-25, 18.00-33

1.2   Star Rating, Ply Rating and Load Index.

The load capacity of a tire is indicated by the star rating( in case of radial tire) and the ply rating (in case of bias tire).

The load index is applied in countries where the ETRTO standards are used.

1.3   Overall Diameter (OD)

“Overall Diameter” is twice the section height of a new tire, plus the nominal rim diameter, including 24-hour inflation growth.

1.4   Overall Width (OW)

“Overall width” is the width of a new tire, including 24-hour inflation growth, and including protective side ribs, bars or decorations.

1.5   Section Width (SW)

“Section Width” is the width of a new tire, including 24-hour inflation growth and including normal sidewalls, but nor including protective side ribs, bars, or decorations.

1.6   Static Loaded Radius and Width (SLR, SLW)

“Static Loaded Radius” is the shortest distance from the axle center to the contact surface of a tire and “Static Loaded Width” is the overall width of a tire, mounted on the approved rim at the specified inflation pressure and placed still and vertically on a flat board, and loaded with the specified load.

1.7   Original Tread Depth (OTD)

“Original Tread Depth” is the tread depth of a new tire measured at the point of tread-indicator where available or one-fourth the width of the tire crown section from the crown center, including 24-hour inflation growth.

4. 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 
5. Fitting Guide

 PLEASE CONSULT.