Opel is usually considered for making sensible cars but there has been a few wacky ideas in their history and the Opel GT2 from 1975 suits that category perfectly. Back then there was a lot demand more comfort, style, fuel consumption and safety.
They came up with a solution and that was the GT2 which had a futuristic wedge look and sliding doors with no visible rails which were later used on the Peugeot 1007. You could open the door by pressing a mechanism located in the rear view mirror.
Although those wasn't the only futuristic thing on this concept car. The interior had ventilated seats, digital gauges instead of the conventials instruments and a onboard computer. The Opel GT2 concept was based on the Monta and Ascona.
But that syliong wasn't just for looks it also helped it gain 0.326 drag coefficient which was at least 16.6% better than the Opel experimental GT. It was reported to have a 1900 cc 4-cylinder engine that could produce 105 hp.
With a top speed of 200 km/h (124 mph) which was pretty fast back then. Also the GT2 was able to do 7 to 7.5 litres per 100 km (62 miles), that might not be much today but back in 1975 it was pretty good by those standards.