In driving my commute to work I have the option of a much longer highway route or a bunch of stop lights. By my rough calculations though based on the much better gas mileage in my car, I would save gas by driving the long way around. I've been wanting google to do this calculation for me in google maps (I've been bugging my friends that work there...) but to no avail.
However Garmin has announced that they have created "ecoroute" a route planning algorithm for nuvi 200 and 700 series GPS units
Garmin's website ecoroute page says:
"users will have a “less fuel” option as a routing preference. When selected, nüvi will determine the best route for saving gas."
"Fuel Report tracks fuel usage over time, and Mileage Report monitors mileage and fuel usage on a per-trip basis. You can also customize your nüvi by entering vehicle type, your car’s fuel economy and the current price of gas."