What 235/55R18 means
The inputs are 235 mm nominal section width, a 55-percent aspect ratio, and an 18-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 235 × 55 ÷ 100 = 129.25 mm. Adding two sidewalls to the 457.2 mm rim gives 715.7 mm (28.18 in) of nominal outside diameter.
A calculated reference
The 129.25 mm sidewall is a mathematical result from 235 × 0.55. The published comparison with 235/60R18 keeps the first and last inputs constant, making the effect of the five-point ratio change explicit.
Using a perfect-circle model, circumference is 2248.44 mm, equivalent to 444.75 revolutions per kilometer or 715.76 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 235/55R18, article 15576590000. 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