Sixty-six drivers participated in a test of their knowledge and understanding of instrument panel displays. They were asked about specifications for their vehicles (e.g., engine temperatures), viewed numerous slides of instrument clusters and said what was wrong (e.g., low fuel) and what they would do (e.g., stop at the next gas station), and ranked displays from most to least understandable.The data showed that -- (1) Many drivers knew little about their vehicles. (2) For engine functions, drivers were more likely to understand moving pointer than numeric displays. Pointer alignment, color-coding, and labeling the normal zone all greatly improved understanding of engine displays. (3) Drivers understood all of the existing labeling schemes for analog fuel displays but had varying degrees of difficulty with digital fuel displays.