Obama's claim about Japanese cars getting more than 45 mpg involves the fuzzy science of measuring fuel economy, complicated by differing standards in each nation. Obama's campaign says his number comes from a 2004 study by the Pew Center on Global Climate Change that calculated Japanese cars would get 46 mpg if Japan followed the same test methods as the U.S. (Obama accurately says the U.S. mandate has been 27.5 for 20 years.)
Obama's claim is technically correct. But the Pew numbers exaggerate the differences between the countries because those estimates do not reflect real-world conditions, according to Lee Schipper, director of research at the World Resources Institute's Center for Transport and Environment.
He says the Japanese fleet actually averages about 23 mpg on the road today, but is improving. The U.S. number is only slightly lower, about 21, but has been stagnant. The real-world numbers are closer because, despite more fuel-efficient cars in Japan, heavy traffic congestion erases much of the efficiency.
Schipper said Obama "is correct that Japan's fuel efficiency is higher and improving more rapidly, but his number is inaccurate. The number he used doesn't really represent a realistic on-road achievement."
Yet Schipper said he doesn't fault Obama for the confusion. "Everybody makes this mistake. What matters is what cars really get on the road, how long it takes to change the entire fleet and how much people drive." (Japanese drivers drive about one-third as many miles per year as U.S. drivers.)