What 245/45R18 means
The inputs are 245 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 245 × 45 ÷ 100 = 110.25 mm. Adding two sidewalls to the 457.2 mm rim gives 677.7 mm (26.68 in) of nominal outside diameter.
A calculated reference
The 110.25 mm nominal sidewall is determined by width and ratio. The curated comparison from 245/40R18 adds 12.25 mm to each sidewall on the same 18-inch rim, giving 24.5 mm more outside diameter. It isolates the aspect-ratio change without changing nominal width.
Using a perfect-circle model, circumference is 2129.06 mm, equivalent to 469.69 revolutions per kilometer or 755.9 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 245/45R18, article 15577580000. 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