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.Five different fuels - comprising one Primary Reference Fuel (PRF) blend, one Toluene Standard Fuel blend and three full boiling range, multi-component fuel blends resulting in sensitivities ranging from 0 to 12.3 - were evaluated. Seven different vehicles including two naturally aspirated Port Fuel Injected (PFI), a turbocharged and intercooled PFI and four turbocharged and intercooled Direct Injection Spark Ignition (DISI) vehicles were used for the evaluation. Engine performance in terms of vehicle acceleration (50 to 110 km/hr) was measured while simultaneously recording engine operating parameters such as ignition timing and boost pressure. Vehicle performance indicated that, as expected, the high sensitivity fuels provided performance benefits to the boosted and intercooled engines.The naturally aspirated PFI vehicle was not obviously knock limited at the high altitude test site, while a different naturally aspirated vehicle tested at sea level was and indicated generally higher levels of ignition retard for the less sensitive fuels, although this trend did reverse at higher engine speeds. The boosted PFI vehicle (tested only at altitude) used ignition retard only to control knock except for the zero sensitivity fuel (PRF) for which boost was also reduced at mid engine speeds. The boosted DISI engines relied on both ignition retard and, to a lesser extent, boost to control knock. The boosted DISI engines compensated fully for altitude. At sea level, the zero sensitivity fuel produced consistently lower boost pressures than all of the other fuels in the boosted DISI vehicles throughout the speed range. However, an interesting phenomenon occurred at altitude whereby the zero sensitivity fuel resulted in significantly higher boost pressures during the initial part of the accelerations, moderating to lower boost pressures in the middle to higher vehicle speeds. This occurred for both the boosted DISI and boosted PFI vehicles. This is thought to be as a direct consequence of the higher exhaust enthalpy available for turbine work which would result from the retarded ignition timing and allowing a more rapid build-up of boost pressure prior to boost pressure control becoming active. The knock control strategies appeared to respond as expected, confirming that the higher sensitivity fuels provide greater resistance to knock in these engines.