The technology relates to enhancing the operation of autonomous vehicles. Extendible sensors are deployed based on detected or predicted conditions around a vehicle while operating in a self-driving mode. When not needed, the sensors are fully retracted into the vehicle to reduce drag and increase fuel economy. When the onboard system determines that there is a need for a deployable sensor, such as to enhance the field of view of the perception system, the sensor is extended in a predetermined manner. The deployment may depend on one or more operating conditions and/or particular driving scenarios. These and other sensors of the vehicle may be protected with a rugged housing, for instance to protect against damage from the elements. And in other situations, deployable foils may extend from the vehicle's chassis to increase drag and enhance braking. This may be helpful for large trucks in steep descent situations.