In-car devices that use audio output have been shown to be less distracting than traditional graphical user interfaces, but can be cumbersome and slow to use. In this paper, we report an experiment that demonstrates how these performance characteristics impact whether people will elect to use an audio interface in a multitasking situation. While steering a simulated vehicle, participants had to locate a source of information in a short passage of text. The text was presented either on a visual interface, or using a text-to-speech audio interface. The relative importance of each task was varied. A no-choice/choice paradigm was used in which participants first gained experience with each of the two interfaces, before being given a choice on which interface to use on later trials. The characteristics of the interaction with the interfaces, as measured in the no-choice phase, and the relative importance of each task, had an impact on which output modality was chosen in the choice phase. Participants that prioritized the secondary task tended to select the (faster yet more distracting) visual interface over the audio interface, and as a result had poorer lane keeping performance. This work demonstrates how a user's task objective will influence modality choices with multimodal devices in multitask environments.