(PopularScience) For years, automakers have worked to push fuel economy beyond 100 miles per gallon. Reaching that mark typically meant three things: cutting weight, maximizing aerodynamics, and improving powertrain efficiency. In 1999, Volkswagen engineers got close with the Lupo 3L, a three-cylinder coupe that could go 78.4 miles on one gallon of diesel. Not satisfied, VW tasked star engineer Ulrich Hackenberg, whose résumé includes work at Bentley and Bugatti, with breaking the 100mpg barrier. Hackenberg”s team crushed that goal—and then some. In tests, their new XL1 got a mind-bending 261 mpg.