What changes between these markings
Holding the 245 mm width and 18-inch rim constant makes the five-point aspect-ratio change easy to isolate. Each sidewall grows by 12.25 mm, so nominal outside diameter increases by 24.5 mm. The full-diameter percentage is calculated from 653.2 mm, rather than from the aspect-ratio number.
With 245/40R18 as original and 245/45R18 as new, nominal diameter changes from 653.2 to 677.7 mm: +24.5 mm (+3.75%). Nominal section width changes by 0 mm and sidewall height by +12.25 mm per side.
Circumference and theoretical speed
Calculated circumference changes from 2052.09 to 2129.06 mm. Revolutions per kilometer change from 487.31 to 469.69, a −3.62% 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 103.75 km/h for the new size. At an indicated 60 mph it gives 62.25 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 245/40R18 is the original denominator. Reversing the inputs gives a −3.62% diameter change and 96.38 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 +12.25 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 CivicX author asks about changing the aspect ratio from 40 to 45 at a stated 245 mm width on 18-inch wheels. The discussion explicitly identifies both complete markings. TireSpan uses the question to select this comparison and does not adopt the replies’ percentage thresholds or handling claims.
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
- ExtremeContact DWS06 Plus — Specifications — Continental Tire
- AltiMAX RT45 — General Tire
- 40 or 45 sidewall on 18s? — CivicX
- Tire glossary — Michelin