This NPR story about a study which rewarded drivers for keeping within the speed limit has some resonances here. Quite a few Prius drivers have reported that their driving habits have been changed by trying to beat the mileage "game" presented by the various displays, which effectively provides an immediate reward in the form of lower fuel costs. This constant feedback from our cars encourages moderate acceleration, smooth stops, and keeping within the speed limit, all of which contribute to gas savings. From the story: "Researchers have found that a small GPS-based device that constantly measures the speed of a car against the posted speed limit can slow drivers down, if it's paired with the right financial incentive. "In a 'proof-of-concept' experiment partly funded by the National Highway Traffic Safety Administration, researchers found that drivers slow down dramatically when they are promised a $25 prize at the end of each week of safe driving. "The catch: Every time the test subjects went five to eight mph above the speed limit, they lost 3 cents from their prize. And every time they went nine or more mph above the speed limit, they lost 6 cents." And: "Unlike the way insurance rebates are currently configured, the devices Reagan and his colleagues used gave drivers real-time feedback — every time drivers turned off the ignition, they were informed about the 'cost' of that trip."