What changes between these markings
The second marking is 20 mm wider, yet its nominal sidewall is 3.25 mm shorter. The smaller 40-percent ratio outweighs the extra width in the height calculation, giving a 6.5 mm reduction in outside diameter on the same 18-inch rim.
With 225/45R18 as original and 245/40R18 as new, nominal diameter changes from 659.7 to 653.2 mm: −6.5 mm (−0.99%). Nominal section width changes by +20 mm and sidewall height by −3.25 mm per side.
Circumference and theoretical speed
Calculated circumference changes from 2072.51 to 2052.09 mm. Revolutions per kilometer change from 482.51 to 487.31, a +1% 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 99.01 km/h for the new size. At an indicated 60 mph it gives 59.41 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 225/45R18 is the original denominator. Reversing the inputs gives a +1% diameter change and 101 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 −3.25 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 Bimmerpost question names this exact size pair and also discusses run-flat construction. The size calculation can compare the markings, but it cannot reproduce the behavior of different tire constructions.
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
- ExtremeContact DWS06 Plus — Specifications — Continental Tire
- Need advise 225/45R18 vs 245/40R18 Also RF vs NRF — Bimmerpost
- Tire glossary — Michelin