In light of increasingly stringent CO2 emission targets, Original Equipment Manufacturers (OEM) have been driven to develop engines which deliver improved combustion efficiency and reduce energy losses. In spark ignition engines one strategy which can be used to reach this goal is the full utilization of fuel octane number.Octane number is the fuel´s knock resistance and is characterized as research octane number (RON) and motor octane number (MON). Engine knock is caused by the undesired self-ignition of the fuel air mixture ahead of the flame front initiated by the spark. It leads to pressure fluctuations that can severely damage the engine. Modern vehicles utilize different strategies to avoid knock. One extreme strategy assumes a weak fuel quality and, to protect the engine, retards the spark timing at the expense of combustion efficiency. The other extreme carefully detects knock in every engine cycle and retards the spark timing only when knock is detected. Therefore as fuel octane number improves, the spark is advanced to the knock boundary of that fuel; a process known as knock limited spark advance or KLSA. By employing this strategy the engine is always able to operate at the highest efficiency the fuel permits.Over the last 20 years Shell has been active in measuring the octane appetite of modern gasoline engines and has published extensively in this area. The latest fleet test program evaluated the octane response of 20 modern gasoline vehicles equipped with a wide variety of engine technologies. Using a selection of vehicles targeting the most popular models and brands across Europe in 2013, vehicle performance and ignition timing were measured in response to fuels with different octane qualities at wide open throttle acceleration and different steady state conditions.The majority of the 20 vehicles showed performance benefits when running on higher RON fuels. The highest statistically significant (95% confidence) full speed gated acceleration time benefit was 1.9%, whilst the highest power benefit was 3.9% at 2500 rpm for a RON98 versus RON95 comparison. Comparing the different steady state conditions, greatest benefits were generally found at 2500 rpm; an engine speed that is highly representative of real-world driving.