So I know we've got this wiki on maximizing your fuel economy with traditional fuels as well as this one for hybrid vehicles. But this is a pretty crazy statistic from AutoblogGreen:
Incorrect tire pressure is responsible for the production of an additional 18.4 metric tons of CO2 which end up in the atmosphere each year. This is one of the findings in a report by the FIA (International Automobile Federation) and Bridgestone partnership. We can translate the figure in to wasted fuel as well: about 8 billion liters of fuel wasted per year. Moreover, 55 million tires per year are thrown away because of premature wear and tear due to incorrect pressure. For instance, when running at 90 km/h (60 mph) a car with tire pressure that is 1 bar below the recommended figure burns from one to three percent more fuel.