The advent of 3D displays offers Human-Machine Interface (HMI) designers and engineers new opportunities to shape the user's experience of information within the vehicle. However, the application of 3D displays to the in-vehicle environment introduces a number of new parameters that must be carefully considered in order to optimise the user experience. In addition, there is potential for 3D displays to increase driver inattention, either through diverting the driver's attention away from the road or by increasing the time taken to assimilate information. Manufacturers must therefore take great care in establishing the ‘do’s and ‘don’t's of 3D interface design for the automotive context, providing a sound basis upon which HMI designers can innovate. This paper describes the approach and findings of a three-part investigation into the use of 3D displays in the instrument cluster of a road car, the overall aim of which was to define the boundaries of the 3D HMI design space. A total of 73 participants were engaged over three studies. Findings indicate that users can identify depth more quickly and accurately when rendered in 3D, indicating potential for future applications using the depth dimension to relay information. Image quality was found to degrade with increasing parallax and indications of a fatigue effect with continued exposure were found. Finally, a relationship between minimum 3D offset, parallax position and object type was identified.