Choose the original size deliberately
Every signed change is the new value minus the original value. A percentage divides that change by the original value. Swapping the sizes reverses the absolute difference, but the percentage must be recalculated with its new denominator.
The default comparison
215/55R17 has a nominal diameter of 668.3 mm; 225/45R18 has a nominal diameter of 659.7 mm. The new size is 8.6 mm smaller in diameter, a change of −1.29%. At an indicated 100 km/h, the circumference-ratio model gives a theoretical 98.71 km/h. This is an estimate from nominal sizes, not a measurement of road speed.
Two alignments answer different questions
Center alignment makes the radial difference easy to see around a common center. Ground alignment places both circular outlines on the same baseline. Neither image models a tire’s loaded contact patch or the clearance around a moving suspension.
A comparison does not approve a replacement
A similar diameter cannot establish wheel compatibility, load capacity, speed rating or vehicle fitment. Keep those checks separate and use the applicable vehicle and tire manufacturer information.
Sources and further reading
- Tire Markings Explained — Michelin
- Tires — National Highway Traffic Safety Administration