Explanation of Truck's Tire Size

by Tim Plaehn

Understanding truck tire sizes is important for the safe transport of goods and people. Trucks and buses carry many thousands of pounds, and the tires must be correctly chosen to safely carry the loads. Transmission gearing and road speed are also affected by the tires selected.

Types

Truck tire sizes follow a standard formula. Standard profile will look like 11R22.5G and low-profile will have the form 275/80R22.5G. The 11 or 275/80 gives the section width and aspect ratio. "R" is for radial tires, bias ply tires will have a dash. The 22.5 is the wheel diameter; the "G" is the load rating. Section width is the widest point of an unloaded tire, and the section height is from the wheel rim to the outside of the tire.

Aspect Ratios

Truck tires sizes starting in 8, 9, 10, 11 and 12 are standard profile tubeless with the tire body width in inches. Standard profile tires have an aspect ratio of 88 percent. The height of the tire from rim to tread will be 88 percent of the width. An 11R22.5 tire will be approximately 11 inches wide and 9.5 inches high. Low profile tires show the width in millimeters and the aspect ratio. A 275/80R22.5 tire is 275 millimeters wide 220 millimeters high. Typical low aspect truck tire ratios are 70 percent, 75 percent and 80 percent. Tube-type tires are given widths as 9.00, 10.00 or 11.00, as in 1000R20, pronounced "ten hundred R twenty." Tires requiring inner tubes have an aspect ratio of 98 percent.

Wheels

The wheel size says a lot about the tire. All wheel sizes are the wheel diameter in inches. Whole number sizes indicate multi-piece wheels that require an inner tube. Most common tube wheel sizes are 20, 22 and 24 inches. Wheels with half-inch sizing are single piece for tubeless tires. Common sizes are 19.5, 22.5 and 24.5 inches. Truck wheels also come in several widths; a tire size chart should be consulted to match the tire size and wheel width.

Load Range

The final letter in the tire size is the load range, which indicates how much weight the tire can carry. The popular load range "G" in 11R22.5 and low-profile 22.5 can support 6,175 lbs. inflated to 105 psi in a single tire application. Tires mounted as duals will have a slightly lower load rating. Also, tires will have different load capacity at different inflation pressures. The tire manufacturer will provide a chart with all of the necessary data. The big load range "L" tires on the front axle of dump trucks and cement mixers can carry loads of over 12,000 lbs. per tire.

History

Modern tubeless, radial truck tires have become the standard on trucks only since the late 1980s. Before then, the most common sizes were 10.00-20 and 10.00-22. The tubeless radial equivalents of these tires are 11R22.5 and 11R24.5. The low-profile tires became common in the late 1990s on highway trucks. Low profile offers less weight and better performance.

About the Author

Tim Plaehn has been writing financial, investment and trading articles and blogs since 2007. His work has appeared online at Seeking Alpha, Marketwatch.com and various other websites. Plaehn has a bachelor's degree in mathematics from the U.S. Air Force Academy.