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The effect of fuel anti-knock quality on power and acceleration performance is studied in twenty-three European and Japanese cars equipped with knock sensors. The anti-knock quality of a fuel in a given car and operating condition is defined by its octane index OI = RON - KS where K is a constant for that condition and S is the sensitivity, (RON-MON), and RON and MON are the Research and Motor Octane numbers respectively. The higher the octane index, the better the antiknock quality of the fuel and the better the power and acceleration performance. K is often assumed to be 0.5 so that OI =(RON+MON)/2. However, it is found that in most cases considered here, K is negative so that for a given RON, a fuel with higher sensitivity (lower MON) has better anti-knock quality and better performance. Even when K is not negative it has a small (< 0.2) positive value so that MON contributes much less to fuel anti-knock quality than generally assumed.