What changes between these markings
Increasing width by 10 mm does not compensate for reducing the aspect ratio from 45 to 40. Each nominal sidewall becomes 8.25 mm shorter. With the same 19-inch rim, that produces a 16.5 mm smaller diameter.
With 245/45R19 as original and 255/40R19 as new, nominal diameter changes from 703.1 to 686.6 mm: −16.5 mm (−2.35%). Nominal section width changes by +10 mm and sidewall height by −8.25 mm per side.
Circumference and theoretical speed
Calculated circumference changes from 2208.85 to 2157.02 mm. Revolutions per kilometer change from 452.72 to 463.6, a +2.4% 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 97.65 km/h for the new size. At an indicated 60 mph it gives 58.59 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 245/45R19 is the original denominator. Reversing the inputs gives a +2.4% diameter change and 102.4 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 −8.25 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 MOTOR-TALK question starts from 245/45R19 and asks about 255-width alternatives, with 255/40R19 explicitly discussed. The page provides a numerical reference without adopting the forum’s handling or approval 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
- AltiMAX RT45 — General Tire
- ExtremeContact DWS06 Plus — Specifications — Continental Tire
- Felge Serres — mögliche Reifengrößen? — MOTOR-TALK
- Tire glossary — Michelin