The overall objectives of this study are to establish the relationship between a spark ignition, or Otto cycle, engine energy efficiency and the octane number of the fuel through a comprehensive review of recently published literature. The efficiency of the ideal Otto cycle is a function of the compression ratio, but increasing compression ratio is limited by the onset of knock, which can be prevented by increasing fuel octane number. Hence, in an ideal case, there is a direct connection between engine efficiency and fuel octane number. In the real world, other factors also contribute to the relationship and spark timing is the primary control variable that affects both knock and efficiency. This analysis explores the relationship between efficiency and octane number. Since engine efficiency is a function of the engine operating point in terms of speed and load, engine based data at single or a limited number of load and speed points may provide only a partial view on how engine efficiency changes would affect vehicle fuel economy over a driving cycle. In addition, the issues of SI engine efficiency and fuel octane number are made more complex by the fact that absolute engine efficiency is itself a function of other technology such as the type of fuel system, engine aspiration and combustion chamber design, and an evaluation must address all of these issues. The evaluation was based on a review of about 60 papers published between 1987 and 2015. The literature review found that the relationship between octane number and efficiency was influenced by a number of intermediate variables covering engine type, operating condition, and fuel formulation. In general, octane number increases appear to be most useful for turbocharged engines and least useful for naturally aspirated Miller Cycle engines.