Back in 2002, Volkswagen unveiled its concept for a super-efficient one-liter car. The prototype, now dubbed the XL1, has been in development ever since, but now appears to be edging closer to production. Spotted undergoing hot-weather testing by our trusty spy photographers, the 235-mpg prototype packs an 800cc two-cylinder turbo diesel injection engine delivering some 50 horsepower and 88 lb-ft of torque, paired to a 30 hp electric motor. Both engines channel their power to the rear wheels via a 7-speed DSG transmission.

Built for mileage, not performance, the aerodynamically-optimized XL1 has a top speed limited to 99 mph, a 0-62 mph time of 11.9 seconds and can travel up to 24 miles in all-electric mode. Open the gullwing doors and the interior remains close to that shown in the concept car at the Qatar Motor Show in 2011. As does the exterior, save for the very narrow rear track and wheel wells. Expect full wheel covers to aid aerodynamics when the XL1 hits the market in 2013.