What changes between these markings
The width stays at 225 mm while the rim grows by 25.4 mm and the two sidewalls together shrink by 22.5 mm. Those opposing changes leave only a 2.9 mm increase in nominal diameter. The close outside diameters do not mean the tires mount on the same rim.
With 225/45R17 as original and 225/40R18 as new, nominal diameter changes from 634.3 to 637.2 mm: +2.9 mm (+0.46%). Nominal section width changes by 0 mm and sidewall height by −11.25 mm per side.
Circumference and theoretical speed
Calculated circumference changes from 1992.71 to 2001.82 mm. Revolutions per kilometer change from 501.83 to 499.54, a −0.46% 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 100.46 km/h for the new size. At an indicated 60 mph it gives 60.27 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 225/45R17 is the original denominator. Reversing the inputs gives a −0.46% diameter change and 99.54 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 +1.45 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
An Audi Klub Polska author explicitly asks about the comfort difference between 225/45R17 and 225/40R18. TireSpan uses that original question as comparison context and does not reuse the replies as evidence of ride quality or vehicle suitability.
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
- Kto jezdzi na felgach 18? — question comparing 225/45R17 and 225/40R18 — Audi Klub Polska
- Tire glossary — Michelin