What 225/45R18 means
The inputs are 225 mm nominal section width, a 45-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 225 × 45 ÷ 100 = 101.25 mm. Adding two sidewalls to the 457.2 mm rim gives 659.7 mm (25.97 in) of nominal outside diameter.
A calculated reference
This is the new size in TireSpan’s reference comparison against 215/55R17. The 18-inch rim is larger while the calculated outside diameter is smaller. Its comparison with 245/40R18 also shows why greater width can accompany less sidewall height.
Using a perfect-circle model, circumference is 2072.51 mm, equivalent to 482.51 revolutions per kilometer or 776.52 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 225/45R18, article 15577440000. 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