Findings from an intermediate ambient temperature vehicle driveability study for isobutanol gasoline blends are reported. The pattern for the study was Coordinating Research Council Project CM-138-02, which investigated the effects of ethanol on cold-start/warm-up performance and Driveability Index. Objectives of the present study were: (a) to evaluate the efficacy of the current ASTM Driveability Index (DI) in predicting cold-start and warm-up driveability performance for isobutanol gasolines and (b) if required, identify modifications to the DI definition and specification limits for isobutanol blends. The test fuel matrix included fifteen fuels with nominal vapor pressures of 55 kPa (8 psi) at DI levels of 1150, 1200, 1250, and 1300 and isobutanol concentrations of 0, 16, and 24 volume percent. Twelve port- and direct-fuel-injected vehicles, which included US Tier 2 passenger cars and light-duty trucks from model years 2005 through 2008, were used to evaluate the test fuels. Cold-start and warm-up driveability tests were conducted at 4°C (40°F) following CRC E28 procedures as modified for all-weather chassis dynamometer implementation. As found in previous studies, cold-start and warm-up driveability performance deteriorated sharply for all fuels at DI levels above 1250. The present study also found the current DI relationship insufficient for predicting the driveability performance of isobutanol-blended fuels. A new driveability performance model was developed which successfully compensates for isobutanol and collapses to the traditional DI relationship when isobutanol is absent. Current ASTM specifications for maximum DI were found to be appropriate for the new relationship as well. Results from this study will provide useful guidelines for blending isobutanol fuels with good cold-start and warm-up driveability performance.