Spark-ignition (SI) engine efficiency is typically limited by fuel auto-ignition resistance, which is in practice described by the Research Octane Number (RON) and the Motor Octane Number (MON). This paper assesses the auto-ignition resistance (as measured in a modern directly injected spark ignition (DISI) engine) of three compositionally dissimilar gasoline-like fuels with identical RON values and varying or non-varying MON values. The three gasolines are an alkylate blend (RON=98, MON=97), a blend with high aromatic content (RON=98, MON=88), and a blend of 30% ethanol by volume with gasoline (RON=97, MON=87). The goal of this work is to assess whether fuel properties, in this case RON and MON ratings coupled with a fuel’s heat of vaporization, are sufficient to describe the anti-knock behavior of varying gasoline formulations in modern engines. The preliminary findings of this work are that RON and MON are sufficient to describe the auto-ignition resistance of a fuel to a degree such that engine efficiency varies less than 1%. However, differences among the fuels are measurable, and ethanol content (30% by volume) is found to improve thermal efficiency and load at higher intake pressures when compared to the high-aromatic fuel of identical RON and MON. Specifically, the higher heat of vaporization of E30 provide a significant benefit in terms of knock-limited CA50. Further, the fuels exhibit dissimilar behavior with regards to auto-ignition intensity, heat-release behavior, and sensitivity to pressure and temperature. Yet these dissimilar behaviors combine to produce similar anti-knock qualities and engine performance for naturally aspirated operation.