February 8, 2020
Have you ever felt like you needed a decoder ring from your childhood to understand numbers and letters imprinted on tires? It’s a well-thought-out system, where every number and letter, even a dash, has a standardized meaning.
Tire experts Scott Sloan, ag products manager for Titan and Goodyear farm tires, and Brad Harris, manager of global field engineering for Firestone Ag, give examples.
For radial tractor tires, Sloan offers this example of a common size: 800/70R38. The 800 is width of the tire in millimeters, and 70 is the aspect ratio, which is the ratio of the height to the width of the tire. In this case, the height of the tire’s sidewall is 70% of its width. The R stands for radial. The 38 is the wheel diameter in inches. If the tire is an IF, VF or LSW type of construction, those letters would precede the size, such as IF800/70R38 for an “increased flex” tire.
CFO after the wheel size number stands for “cyclic field operation,” which means it can handle loads 55% heavier than IF tires under 10 mph or 30% heavier than IF tires at 20 mph or less, Sloan says. This is most useful for combines and grain carts.
A number immediately preceding the speed rating letter is the load index, which designates the maximum load-carrying capacity of a tire at its maximum air pressure and speed, Sloan explains. For example, a 151B load-speed rating would mean 7,600 pounds at 23 pounds per square inch max at 31 mph. That information comes from a table. Consult load and inflation tables provided by the tire manufacturer.
Harris provides more examples of tire designations and what they mean:
Standard marked radial tractor tire: 18.4R42 154B. The 18.4 is the width of the tire in inches, R indicates radial, 42 is the rim diameter in inches, 154 is the rated load of 8,250 pounds, and B signifies the maximum rated speed of 30 mph.
Implement tire: 11L-15 8-ply. The 11 is the width of the tire in inches, the L indicates the tire has a low sidewall height, the dash indicates bias construction, 15 is the rim diameter in inches, and 8-ply states the rated load is 2,540 pounds.
Radial implement tire: IF280/80R15 128D. IF indicates the increased flexion load formula for radial tires, 280 is width of the tire in millimeters, 80 is the sidewall aspect ratio, meaning the tire sidewall height is 80% of the tire width. The R indicates radial, 15 is the rim diameter in inches, 128 is the rated load of 3,960 pounds, and D is maximum speed of 40 mph.
Narrow, high-clearance sprayer tire: VF380/90R46 173D. VF means it’s a very high flexion-designed radial tire, 380 is width in millimeters, 90 is sidewall aspect ratio, R stands for radial, 46 is rim diameter in inches, 173 is the rated load index of 14,900 pounds according to the manufacturer’s table, and D means maximum rated speed is 40 mph.
Combine and grain cart tires: IF1250/50R32 CFO 201B. It’s an IF design, 1,250 millimeters wide with a 50-sidewall aspect ratio, radial tire. The 32 denotes rim diameter in inches. CFO means it’s approved for cyclic field operations, where weight carried fluctuates widely. The 201 signifies rated load of 32,000 pounds, and B means maximum speed is 30 mph.
Learn about tire tread terminology in this related article.
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