What 225/40R18 means
The inputs are 225 mm nominal section width, a 40-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 × 40 ÷ 100 = 90 mm. Adding two sidewalls to the 457.2 mm rim gives 637.2 mm (25.09 in) of nominal outside diameter.
A calculated reference
The 90 mm sidewall is 11.25 mm shorter than for 225/45R17. The larger 18-inch rim offsets that reduction, leaving the overall nominal diameter 2.9 mm larger. This curated comparison illustrates why a rim-size change should be evaluated together with the sidewall term.
Using a perfect-circle model, circumference is 2001.82 mm, equivalent to 499.54 revolutions per kilometer or 803.94 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 225/40R18, article 15577710000. 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