The ultra-fuel-efficient Volkswagen XL1 has been spied undergoing hot-weather testing in Europe, ahead of the plug-in diesel hybrid’s 2013 launch.

The images reveal two XL1 prototypes – one silver and one matt black – both of which appear almost identical to the sub-1.0L/100km XL1 concept that was first unveiled at the 2011 Qatar motor show early last year.

Although the spy shots show the prototype twins with a conventional visible wheel design, the rear wheels of both vehicles are set back behind the exterior panels indicating that the production model will retain the concept’s flush rear wheel covers that improve aerodynamic efficiency.

The futuristic front-hinged scissor doors, LED headlights and tail-lights and minimalistic interior will also carry over into the production car.

Less clear at this stage is whether the production model will inherit the concept’s rear-view side cameras and internal digital displays or trade them for traditional side mirrors, as seen on the black prototype (above).

The Volkswagen XL1’s powertrain promises to be no less technologically advanced than the concept, however.

Like the concept, the production model is expected to team a 35kW 800cc two-cylinder diesel engine, a 19kW electric motor and a lithium-ion battery pack, with a seven-speed dual-clutch DSG automatic transmission sending drive to the rear wheels.

Despite its modest output, the XL1’s super-slippery 0.186 drag coefficient and anticipated body weight of 800kg means it can accelerate from 0-100km/h in approximately 12 seconds – roughly the same as other light- and small-sized diesel vehicles.

Best of all is its anticipated fuel economy, with Volkswagen targeting a combined cycle rating as close to the concept’s 0.9 litres per 100km figure as possible for the production car.

Volkswagen confirmed late last year that the two-seat plug-in would enter production in 2013, following its anticipated unveiling at an international motor show, potentially Frankfurt in September.