Low-temperature combustion (LTC) engine concepts such as homogeneous charge compression ignition (HCCI) offer the potential of improved efficiency and reduced emissions of nitrogen oxide (NOx) and particulates. However, engines can only successfully operate in HCCI mode for limited operating ranges that vary depending on the fuel composition. Unfortunately, traditional ratings such as octane number (ON) poorly predict the auto-ignition behavior of fuels in such engine modes, and metrics recently proposed for HCCI engines have areas of improvement when wide ranges of fuels are considered. In this study, a new index for ranking fuel suitability for LTC engines was defined, based on the fraction of potential fuel savings achieved in the federal test procedure (FTP-75) light-duty vehicle driving cycle. Driving cycle simulations were performed using a typical light-duty passenger vehicle, providing pairs of engine speed and load points. Separately, single-zone naturally aspirated HCCI engine simulations were performed for a variety of fuels in order to determine the operating envelopes for each. These results were combined to determine the varying improvement in fuel economy offered by fuels, forming the basis for a fuel performance index. Results showed that, in general, lower octane fuels performed better, resulting in higher LTC fuel index values; however, ON alone did not predict fuel performance.