Yeah, that's well above the Guinness World Record but the vehicle has been modified and was no longer a 'factory car'. Also: Driving a constant 57 km/h / 35.4 mph
on the German Autobahn is illegal and could not be done today, 41 years later. But it clearly shows that TDI's are efficient (and were so a loooong time ago) and that a low curb weight combined with low roll resistance (aerodynamics are a lesser factor at that speed)are the key to success ...