A variety of factors go into the fuel economy of a car. Although tires are not often the first feature that energy-conscious consumers look into when measuring the gas efficiency of a vehicle, tires do indeed contribute to the way in which a car uses fuel. The role of tires in gas efficiency has been a hot topic of discussion in the past: President-Elect Obama made the claim in April 2008 that tire inflation boosts gas mileage -- a remark that was initially mocked, but turns out to be supported by reports indicating that proper inflation can improve mileage by 3 percent. Other variables in fuel efficiency include tire weight and rubber make. 5 percent of gas is used up by the rolling resistance, or energy output, of a tire. Low-rolling resistance tires perform the best in government fuel economy tests because they require the least amount of push to move. Consumer Reports researched the affects of low-rollers on gas mileage, calculating that about $300 a year can be saved by opting for tires with the best rolling resistance. California led the way for state-wide support of a fuel economy initiative, implementing a gas-efficient tire program in summer 2008 that establishes minimum efficiency standards for passenger cars and light-duty trucks. Of the approximately 26 million passenger vehicles currently registered in California, 21 million vehicles are not running on low-rolling resistent wheels. The California Energy Commission projects that if these cars used tires with a 10% lower rolling resistance, the result would be a "statewide savings of 252 million gallons of fuel, $882 million, and 2.7 million metric tons less CO2 emissions annually."