As well as background noise and the acoustic conditions of the given space, speech intelligibility (SI) is affected by the binaural effect (BE), which is sensitive to the head orientation (HO) of the listener, especially in a small enclosed space such as an automobile. This study uses the speech transmission index (STI) to systematically investigate and predict SI for various HOs of the listener in an automobile. To explore the combined effects of reflections and BE on the auditory perception of speech, several groups of binaural room impulse responses for different speaker locations and HOs are measured in a listening room and an automobile, with the left-side front window (LFW) closed and open, and these are used to calculate the STI. The SI for various configurations is evaluated indirectly using a Chinese STI–SI model. The results show that reflections in the automobile help to increase the STI of the contralateral ear rather than the ipsilateral ear. The LFW is an important reflective boundary but does not always play a dominant role in the STI (SI). Moreover, the STI (SI) can be improved when the listener in the driver seat turns their head inward, i.e., at a negative HO.