U.S. contained ethanol at different proportions, and ethanol production grew so fast that the US became the world's top ethanol producer, overtaking Brazil in 2005. This shift also contributed to a sharp increase in the production and sale of E85 flex vehicles since 2002. Flexible-fuel vehicles by country Brazil Flexible-fuel technology started being developed by Brazilian engineers near the end of the 1990s. The Brazilian flexible fuel car is built with an ethanol-ready engine and one fuel tank for both fuels. The small gasoline reservoir for starting the engine in cold weather, used in earlier neat ethanol vehicles, was kept to avoid start up problems in the central and southern regions, where winter temperatures normally drop below . An improved flex motor generation was launched in 2009 and allowed to eliminate the need for this secondary gas reservoir tank. Another improvement was the reduction of fuel consumption and tailpipe emissions, between 10% to 15% as compared to flex motors sold in 2008. In March 2009 Volkswagen do Brasil launched the Polo E-Flex, the first flex fuel model without an auxiliary tank for cold start. A key innovation in the Brazilian flex technology was avoiding the need for an additional dedicated sensor to monitor the ethanol-gasoline mix, which made the first American M85 flex fuel vehicles too expensive. Brazilian flex cars are capable of running on just hydrated ethanol (E100), or just on a blend of gasoline with 25 to 27% anhydrous ethanol (the mandatory blend), or on any arbitrary combination of both fuels. The flexibility of Brazilian FFVs empowers the consumers to choose the fuel depending on current market prices. As ethanol fuel economy is lower than gasoline because of ethanol's energy content is close to 34% less per unit volume than gasoline, flex cars running on ethanol get a lower mileage than when running on pure gasoline. However, this effect is partially offset by the usually lower price per liter of ethanol fuel. As a rule of thumb, Brazilian consumers are frequently advised by the media to use more alcohol than gasoline in their mix only when ethanol prices are 30% lower or more than gasoline, as ethanol price fluctuates heavily depending on the result of seasonal sugar cane harvests. In March 2003 Volkswagen do Brasil launched in the market the Gol 1.6 Total Flex, the first commercial flexible fuel vehicle capable of running on any blend of gasoline