In light of increasingly stringent CO2 emission targets engine development has been driven to improve combustion efficiency and to reduce energy losses. One strategy to reach this goal is the full utilisation of fuel octane. Octane is the fuel´s knock resistance, which is characterised 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 with 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 in case knock is detected. Therefore as fuel octane improves, the spark is advanced to the knock boundary of that fuel, a process known as knock limited spark advance or KLSA. Thus the engine always operates at the highest efficiency the fuel permits. Over the last 20 years Shell has measured and published extensively on the octane appetite of modern gasoline engines. The latest fleet test program evaluated the octane response of 20 modern gasoline vehicles. Vehicle performance and ignition timing were measured in response to fuels with different octane qualities at wide open throttle and different steady state conditions with a selection of vehicles targeting the most popular models and brands across Europe. The majority of the 20 vehicles showed performance benefits when running on higher RON fuels. The highest full speed gated acceleration time benefit was 4.6%, whilst the highest power benefit was 8.7% at 2500 rpm for a RON98 versus RON91 comparison. Comparing the different steady state conditions, greatest benefits were generally found at 2500 rpm, which is an engine speed that is highly representative of real-world driver behaviour.