Most GPS devices in cars today give the driver two choices: shortest route or fastest route. GreenGPS provides a third option: most fuel-efficient route. With gas prices skyrocketing, many drivers would be happy to spend a few more minutes on the road, or take a different route, if it meant burning less gas. The answer could be the GreenGPS navigation service, now being developed by researchers at the University of Illinois at Urbana Champaign (UIUC), which finds the most fuel-efficient route for your vehicle.