Many new motorcycle tires are sold using metric size designations, while many vintage motorcycles through the early 1980s came equipped with tires measured using inch-based designations. Using basic math and conversion tables or fitment guides offered by tire manufacturers, it is simple to choose a metric-sized tire to replace older inch-designated tires.
Deciphering Tire Size Designations
Inch-based tire designations (e.g., 3.50-18) contain only two pieces of size information, the section width (3.50 inches) and the rim diameter (18 inches). Metric tire size designations (e.g., 110/90-18) contain three measurements, the section width (110 millimeters (mm)), the aspect ratio (90 percent), and the rim diameter (18 inches). The aspect ratio is the ratio of the tire's sidewall height to its section width. A 110/90 tire's sidewall height would be approximately 90 percent of 110 mm, or 99 mm.
Convert Inch-Based Designations to Millimeters
As a rough starting point for comparison, convert inch-based tire size measurements to millimeters. One inch equals 25.4 mm, so a tire with a section width of 3.50 inches would have an equivalent metric width of 3.50 x 25.4 mm/inch = 88.9 mm. This would most closely equate to a metric-sized tire with a 90 mm section width. Both inch-based and metric tire size designations use inches for the rim diameter, so no conversion is necessary for that measurement.
Aspect Ratio/Sidewall Height Considerations
Most older, inch-designated tires had sidewall heights approximately equal to their section width, so a 3.50 inch wide tire would also have a sidewall height of approximately 3.50 inches, or 90 mm.
A 90/90-18 tire would have a sidewall height of 90 mm x 90 percent = 81 mm, about 8 mm (1/3 inch) less than the 3.50-18 tire. Although a small difference, this could affect the handling of the motorcycle or cause speedometer readings to be incorrect.
Consult Tire Manufacturer Fitment Guides
Since aspect ratios over 90 are not typically available, metric tires with a section width equal to older inch-based tires would have a shorter sidewall. Therefore, most tire manufacturer fitment guides recommend a slightly wider 100/90-18 tire, with a sidewall height of 90 mm (100 mm x 90 percent), as a replacement for the 3.50-18.
A 110/90-18 tire would have an approximate width of 4.3 inches (110 mm / 25.4 mm/inch) and a sidewall height of 3.9 inches, so it would be both wider and taller than the 3.50-18 tire.