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greenMeter is an iPhone app which uses the smart phone's accelerometer and user input information on the make of car to calculate the optimal driving speed and style with an eye to saving petrol and therefore money. This also has a positive impact on the carbon dioxide emissions for the journey.
greenMeter is based off code and work from a previous Hunter Research application, gMeter, which calculates acceleration, velocity and other performance related statistics.  This was then modified, and some more calculations added to create an app based on saving fuel and money, and indirectly reducing carbon emissions, rather than an app focused on driving and car performance.
The product can be toggled between US-imperial and metric units. It is also GPS independent.
Dr. Craig Hunter is the man behind this product, and has a history of developing software including winning awards for Mac OSX software in 2004. 
greenMeter was released to the public on September 7 2008. It has been featured in high profile publications such as the New York Times and Wired Magazine. 
It has seen two major updates since its release, though no updates seem to have taken place since late 2009.
greenMeter's creator claims that "greenMeter has likely saved about 2 million gallons of gas, reduced fuel expenses by over $5 million, dropped oil consumption by nearly 20,000 barrels, and prevented almost 47,000 tons of CO2 from entering the atmosphere." 
greenMeter site - hunter.paisite.com
hunter.pairsite.com - creators site