Tire Code

Some farm tires use “millimetric” marking similar to passenger, truck and earthmover tires. Millimetric load and speed ratings are noted in a “service description” which combines a load index and a speed symbol. See below for explanations of these terms. Millimetric marking complies with an ISO standard for agricultural tires and allows tailoring a tire to specific dimensional constraints (diameter and width), load requirements and rolling circumference targets for mechanical front wheel assist tractors.

EXAMPLE: TIRE SIZE: 710/70R38 166A8

“710” TIRE SECTION WIDTH IN MILLIMETERS
Dividing this number by 25.4 gives the width of the tire in inches.

“70” ASPECT RATIO OF TIRE
Just like in a passenger tire, the aspect ratio is the ratio of section height to section width.

ASPECT RATIO = (TIRE SECTION HEIGHT / TIRE SECTION WIDTH) x 100
Dividing this number by 25.4 gives the width of the tire in inches.

“R” RADIAL CONSTRUCTION
“38” NOMINAL RIM DIAMETER IN INCHES
“166” LOAD INDEX
Load indexes are a uniform way to describe the load carrying capacity of the tire. A 166 load index means that the tire can carry 5300 kg or 11700 lbs. Unlike a star rated tire, a load index tire does NOT correspond to a specific inflation pressure. For example, 320/85R34 has a LI (load index) of 133 at 240 kPa (35 psi). A 133 LI means the tire can carry 2060 kg or 4540 lbs. A 480/70R28 has the same LI (same load carrying capacity) but at a different pressure, 120 kPa (17 psi). Always check the handbook to determine the pressure needed to carry the specified load.

“A8” SPEED SYMBOL
The speed symbol determines the maximum speed allowed for the rated load of the tire. From the table on the below, A8 means the maximum speed allowed for the rated load of the tire is 40 km/h or 25 mph.

International Speed Symbols