1.ABSTRACTTo reduce engine exhaust emissions, we have had to deal with this global environmental problem from the fuel side by introducing oxygenated fuels, reducing the RVP and using low aromatics. But when we change the fuel components and distillation, we must take note about how these affect the engine driveability.We have used T50, T90, RVP and so on as the fuel index up to the present. It is possible to characterize the fuel from one aspect, but these indexes don't always represent the real feature of the fuel.In this paper we propose a New Driveability Index (here in after referred to as NDI) that is more realistic and accurate than the other fuel indexes. We used a 1600cc DOHC L4 MPI type engine. We used Model Gasolines and Market Gasolines, see Appendix(1), (2) and (3), and tested them according to the Excess Air Ratio Response Test Method (here in after referred to as λ-R Test) that was suggested in SAE paper #930375, and we calculated the NDI statistically. This NDI is a function of the fuel distillation as shown later. We could express NDI as a linear function of the excess air ratio response test results.NDI is suitable for using with not only hydrocarbon gasolines but also oxygenated gasolines such as those mixed with MTBE, ETBE and Ethanol. Furthermore, we tested two other engine types and confirmed that we could apply this NDI to them.