As SI engines strive for higher efficiency they are more likely to encounter knock and fuel anti-knock quality, which is currently measured by RON and MON, becomes more important. However, the RON and MON scales are based on primary reference fuels (PRF) - mixtures of iso-octane and n-heptane - whose autoignition chemistry is significantly different from that of practical fuels. Hence RON or MON alone can truly characterize a gasoline for its knock behavior only at their respective test conditions. The same gasoline will match different PRF fuels at different operating conditions. The true anti-knock quality of a fuel is given by the octane index, OI = RON −KS where S = RON − MON, is the sensitivity. K depends on the pressure and temperature evolution in the unburned gas during the engine cycle and hence is different at different operating conditions and is negative in modern engines.In this paper we propose that the gasolines are ranked against toluene /n-heptane mixtures (toluene reference fuel, TRF). The gasoline is assigned a Toluene Number (TN), which is the volume percent of toluene in the TRF which matches the gasoline for knock in the CFR RON test. Since TRFs will have comparable sensitivity to gasolines for the same RON, they will have comparable OI at different engine conditions and the TN will describe the knock behavior of the gasoline reasonably well at all different conditions. The paper describes the method and shows supporting experimental data. The other advantage of such a method is that it will enable a better quantitative description of fuels, such as ethanol mixtures, with RON > 100 and hence beyond the maximum of the current octane scale.