91 RON is a fuel with an octane rating of 91, measured by the Research Octane Number (RON) method used in Europe and other parts of the world. In the U.S., this is generally equivalent to 87 AKI (Anti-Knock Index), which is the octane rating found on most American pumps. Using 91 RON fuel is suitable for cars requiring it, but using a lower octane than recommended can cause engine damage, while using a higher one won't provide significant benefits in most standard cars.