What 185/55R15 means
The inputs are 185 mm nominal section width, a 55-percent aspect ratio, and a 15-inch nominal rim diameter. The R identifies radial construction. The width is not a wheel-width measurement, and the rim number is not the outside diameter of the tire.
Nominal sidewall height is 185 × 55 ÷ 100 = 101.75 mm. Adding two sidewalls to the 381 mm rim gives 584.5 mm (23.01 in) of nominal outside diameter.
A calculated reference
A 55-percent aspect ratio at 185 mm produces a 101.75 mm sidewall, close to four inches. The width label does not directly describe tread width or wheel width. Keep those separate when reading a product specification sheet.
Using a perfect-circle model, circumference is 1836.26 mm, equivalent to 544.58 revolutions per kilometer or 876.42 per mile. These are derived from unrounded diameter; actual rolling circumference also depends on the product and operating conditions.
Source and scope
General Tire’s AltiMAX RT45 source includes 185/55R15, article 15576000000. It was checked on October 27, 2026. That record verifies the published marking used here; it does not turn these calculated dimensions into that product’s measured specification or establish availability.
Use the exact product and vehicle requirements for load and speed ratings, permitted rim dimensions, inflation and clearance. Nominal geometry alone cannot verify fitment, braking behavior, handling or safety.
Sources and further reading
- AltiMAX RT45 — General Tire
- Tire Markings Explained — Michelin
- Tire glossary — Michelin