Vehicle road tests, although time consuming, do not usually give accurate fuel consumption data due to the effect of many factors. In this paper, the authors describe a new method for calculating energy consumption during vehicle operation. By using this method, actual vehicle fuel consumption can well be correlated with the calculated energy consumption data.In calculating vehicle energy consumption, the vehicle was driven on a chassis dynamometer according to fixed driving cycles representing urban and suburban operations; as for highway operation, a computer simulation was used to obtain the vehicle energy consumption data. Analyzing this energy consumption data makes it possible to estimate the effects of various parameters, such as rolling resistance, accelerating resistance and so on. In this calculation, consideration of engine mechanical loss is an important factor for accurate and less scattered energy consumption vs. fuel consumption correlation data. This method would be very useful for improving fuel economy.