What changes between these markings
This is TireSpan’s reference example. Increasing the rim from 17 to 18 inches contributes 25.4 mm, but the sidewall pair loses 34 mm. The result is an 8.6 mm smaller nominal outside diameter, despite the larger rim and wider marking.
With 215/55R17 as original and 225/45R18 as new, nominal diameter changes from 668.3 to 659.7 mm: −8.6 mm (−1.29%). Nominal section width changes by +10 mm and sidewall height by −17 mm per side.
Circumference and theoretical speed
Calculated circumference changes from 2099.53 to 2072.51 mm. Revolutions per kilometer change from 476.3 to 482.51, a +1.3% change. Circumference follows diameter in this circular model; revolution counts move in the opposite direction.
At an indicated 100 km/h, the size-ratio calculation gives 98.71 km/h for the new size. At an indicated 60 mph it gives 59.23 mph. Both examples assume calibration corresponds to the original nominal circumference and ignore existing instrument bias and real rolling deformation.
Keep the comparison direction explicit
The published URL keeps one stable order for this pair. Here 215/55R17 is the original denominator. Reversing the inputs gives a +1.3% diameter change and 101.3 km/h at an indicated 100 km/h. Use Swap in the editable tool to inspect that direction; changing only the sign would be inaccurate.
Half of the diameter difference is −4.3 mm of nominal radius change. It is not a measured ride-height change, and the width difference does not locate the installed sidewall without wheel and product information.
Why this pair has a reference page
A published YourMechanic customer question explicitly names 225/45R18 and 215/55R17. TireSpan keeps the numerical comparison while leaving the customer’s vehicle-specific replacement decision outside the model.
The linked manufacturer entries establish the individual size markings. The public discussion establishes an editorial reason to compare them. Neither source role supplies a fitment approval, search-volume estimate, purchase recommendation or universal acceptable percentage. Check complete vehicle and product requirements separately.
Sources and further reading
- AltiMAX RT45 — General Tire
- Can I replace my 225/45R18 tires with 215/55R17 tires? — YourMechanic
- Tire glossary — Michelin