Europe's largest car show has long been a source for fantastic debuts. This year marked a trend reversal for many carmakers, as small-car companies like Mini and Peugeot introduced more comfortable models, while large-car makers scaled things down. Several breathtaking designs were also on display, like Lamborghini's futuristic $1.4-million carbon-fiber Reventon. But the mantra of the show was "fuel efficiency." From hybrid-diesel engines, to ultracompacts, to plug-ins that could power a house in a brownout, Frankfurt's carmakers had gas-sipping on the brain in 2007.