What changes between these markings
The common 55-percent ratio means the wider marking also has a taller sidewall. An extra 10 mm of nominal width adds 5.5 mm per sidewall, giving 11 mm more outside diameter. The nominal radius change is 5.5 mm; it is not an inch of lift or a prediction of loaded ground clearance.
With 225/55R18 as original and 235/55R18 as new, nominal diameter changes from 704.7 to 715.7 mm: +11 mm (+1.56%). Nominal section width changes by +10 mm and sidewall height by +5.5 mm per side.
Circumference and theoretical speed
Calculated circumference changes from 2213.88 to 2248.44 mm. Revolutions per kilometer change from 451.7 to 444.75, a −1.54% 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 101.56 km/h for the new size. At an indicated 60 mph it gives 60.94 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 225/55R18 is the original denominator. Reversing the inputs gives a −1.54% diameter change and 98.46 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 +5.5 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 Reddit author explicitly asks about these two 18-inch, 55-percent markings and raises speedometer, clearance and handling questions. That establishes a reason to compare the dimensions. TireSpan does not repeat the thread’s conflicting fitment statements or treat its claimed height changes as measurements.
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
- Seeking tire Advice: 235/55/18 vs 225/5518 — Reddit r/MazdaCX30
- Tire glossary — Michelin