"What do Detroit, or Stuttgart, or Tokyo have waiting in the wings that will get to the Obama administration's target of 54.5 miles per gallon (mpg) by 2025?"
Detroit, Stuttgart and Tokyo already have the technology. What they lack, and to a great degree what the overwhelming majority of the population also lacks is the will. The "big secret" is to lessen the influence of big oil, both in Detrioit and Washington.
I own a car that is capable of 54.5 mpg - a 1999 VW Jetta TDI. 13 years and 305,000 miles on the odometer and it still gets me to work and back - around 600 miles a week, on 13 gallons of diesel. Yet I still hear the argument at least every other week that "diesel is more expensive than gas" and then the argument that their trusty old Chevy pickup burns (an order of magnitude more) good-n-cheap 87 octane gasoline / gasohol. This leads me to believe that we need to start making basic economics part of the educational system's core curriculum and not just an elective.
OTOH, don't confuse this for an endorsement of the Obama administration. If you want to pay more to waste more energy, the choice is yours. While Obama has been working hard to remove everyone's choices through higher energy prices, the cash for clunkers program, etc. the fact that they've given auto makers 13 years to meet a level we should already be at is evidence that big oil's influence has infiltrated both sides of the aisle to an equal degree.
This will never happen until we get serious about energy independence, and I fear it will take a global catastrophe to bring that about.