Worked example
225/45 R17 and 235/40 R18 are close but not identical. The calculator quantifies the change rather than relying on the size codes looking similar.
Why small percentage changes matter
A larger rolling circumference travels farther per wheel revolution, so the same wheel RPM corresponds to a higher road speed. It also raises the vehicle by roughly half of the diameter difference.
Fitment involves more than diameter
Width, wheel offset, tyre construction and actual measured section width also matter. Use the wheel-offset tool for lateral position and consult vehicle-specific constraints before fitting a non-standard size.