When you look at tire sizing across different vehicles, a noticeable trend stands out: section widths commonly end in 5 and increase in 10 mm increments. Popular sizes include 195, 205, 265, and 315—but why?

A Brief History of Tire Sizing

The way we measure and label tires has evolved dramatically since the early 1900s. Let’s break down each phase in tire sizing development:

High-Pressure Sizing (1900s – 1920s)

Early pneumatic tires used simple sizing: overall diameter followed by section width, such as 30 x 3, where “30” represents the tire’s diameter and “3” the section width. These high-pressure tires, often inflated to 65+ psi, were essential to securing tires on rims.

Numeric Sizing (1920s – 1960s)

As tire designs evolved into “balloon” or lower-pressure tires, sizing began listing section width and rim diameter. A typical size might look like 4.50-21, where 4.50 inches is the section width and 21 inches the rim diameter. Tires of this period had nearly 100% aspect ratios, with sidewall heights equal to section widths. By the 1930s-50s, sizes ending in zero (like 6.70-15 or 7.10-15) indicated around 90% aspect ratios.

The “5” Trend Begins (1960s – Mid 1970s)

By the mid 1960’s, tire sizes transformed again. Any bias ply tire ending in the number five, such as 6.95-14, 7.75-15, etc. featured an 80 to 84 percent aspect

P-Metric Sizing: Why Section Widths End in 5 and Increase in 10 mm Increments

Since the 1970s, the P-metric sizing system, which measures section width in millimeters and aspect ratio as a percentage, has been the standard. Here, widths often end in 5 (e.g., 205/75R15) for several possible reasons:

Aligning with Inch-Based Sizes: It’s possible that when Michelin introduced radial tire technology in the 1940s with the 165 SR 400, the 5-ending width was chosen to align with inch-based sizing that was common at the time. For instance, 165 mm is approximately 6.5 inches, which would have fit well with the inch-based sizes then in use. As radial tires became more popular, the 5-ending widths may have helped create a smoother transition from older inch-based sizing standards.

80% Aspect Ratio Trend: During this period, section widths ending in 5 became associated with an 80% aspect ratio—a trend that established a familiar performance expectation. This consistency must have allowed consumers and manufacturers to quickly link certain widths with specific aspect ratios, simplifying tire selection

Rim Compatibility: One possible reason for the 10 mm increments in tire width could be the need to align with rim sizes, which typically increase in 0.5-inch increments (or 12.7 mm). Standardizing tire widths at 10 mm increments might help maintain a proportional fit with these rim sizes. If tire widths only increased by 5 mm, they would often still require the same rim size, potentially resulting in minimal performance difference.

After digging through tons of articles and info, I couldn’t find a solid answer from any official sources about why tire widths usually end in 5 or go up in 10 mm steps. The reasons I shared—like rim compatibility, matching inch-based sizes, and the 80% aspect ratio trend—are my best understanding based on everything I read. If you know of any other reasons or have come across solid info on this, I’d love to hear it!