The new Brazilian program Inovar Auto coming in 2017 has raised the level of efficiency awareness from vehicle fuel economy in km/L to energy consumption in MJ/km. In this scenario, automakers are implementing several vehicle optimization strategies and new technologies to meet vehicle energy efficiency targets and consequently reduce fuel consumption and exhaust gases emissions. One of these technologies is the START-STOP system that basically turns the engine off when the vehicle is at idle and after some safety and operating trigger conditions are met. A new concept has been developed to evaluate an internal combustion engine start efficiency considering both the electric energy provided by starter motor and the fuel chemical energy required to start and sustain combustion. This concept of start efficiency is of particular interest for Flex Fuel engine where there is a high difference in energy content level between commercial gasohol (E27) and hydrous ethanol (E100) fuels. The method showed to be adequate to parameterize engine start events in addition to conventional parameters like starting time, quality and flare. Furthermore, it has been possible to compare total energy required for starting against idle energy consumption. This paper presents the engine starting efficiency concept and the results of a 1.0L Flex Fuel engine tested with E27 and E100. The effect of start efficiency parameter in function of coolant temperature is also investigated for both fuels.