With gasoline selling for less than the price of a bottle of Evian and SUVs all the rage, fuel economy seems to have fallen off most Americans’ radar screens. But this is the U.S. of A., land of a million subcultures, and one of them is obsessed with the quest for ultra-fuel efficiency. While most of us would be psyched about the hybrid Honda Insight’s 64 miles per gallon, members of the fuel-economy elite scoff at such small potatoes. They talk about much higher figures — try 103 mpg — and swap insider secrets for achieving them: feather the gas pedal, coast downhill, drive barefoot. Cars like the Insight have a feature on the dashboard indicating average mpg, and fanatics play the little green lines like a video game. But the mpg obsession could become all but obsolete when and if hydrogen fuel-cell cars hit the market. That’s still a long way off, but yesterday, DaimlerChrylser’s fuel-cell NECAR 5 completed a 3,000+ mile journey from San Francisco to Washington, D.C., which engineers said was proof that the technology is practical for real-world driving.