Well I wrote a post yesterday, but it doesn't look like it worked. A fuel knock rating of 91 in the US should be adequate for most Ferraris under most driving applications. Most modern engine management computers make adjustments for a small octane difference (i.e. 3 points). If you were to take your Ferrari out for a track day, you are going to want to use the fuel that satisfies the instructions in your manual. You could do this through additives or by using racing fuel. Also, I believe the pump estimate in the US is determined by a different method than the Ferrari manual refers to (formula below). You can check out this great reference
"The correct name for the (RON+MON)/2 formula is the "antiknock index",
and it remains the most important quality criteria for motorists."
"When the combined effects of air temperature and humidity are considered, it is often possible to use one octane grade in summer, and use a lower octane rating in winter."
"A modern engine management system can compensate for altitude, ambient air, temperature, and fuel octane. The management system will also control cold start settings, and other operational parameters."
"In general, modern engine management systems will compensate for fuel octane, and once you have satisfied the optimum octane requirement, you are at the optimum overall performance area of the engine map."
"If the octane is distributed differently throughout the boiling range of a fuel, then engines can knock on one brand of 87 (RON+MON)/2, but not on another brand. This "octane distribution" is especially important when sudden changes in load occur, such as high load, full throttle, acceleration."
--Why Schumacher uses Shell gasoline. Ok, so he likes the hat too.--