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It has been shown that modern spark-ignition engines exhibit greater resistance to auto-ignition for fuels with increased octane sensitivity (RON − MON). This is often presented in terms of the Octane Index, OI = RON − K(RON − MON) with modern engines generally correlating with negative K values. Most of the studies have either presented detailed research type engine tests which show directly the impact on knock limited spark advance (KLSA), or have shown overall vehicle performance benefits and thus have inferred the KLSA response to OI. The aim of this research was to directly measure and compare the actual responses of various engine control parameters of modern production vehicles having different technology levels, to fuels with different OI values derived through different sensitivities. Comprehensive testing was performed at an altitude of over 1500 metres and confirmatory testing performed near sea level.