The gasoline version of the car that used a 4-cylinder engine from a 1980s era Audi only achieved 40 mpg combined. It could reach a top speed of 135 mph and accelerate from 0-60 mph in under 6 seconds. It had a 0.27 drag coefficient and a curb weight of only 1,500 lbs.
Sources:
Avion. (1985). Rocket on wheels: Experimental car set world record for fuel. Retrieved August 6, 2007, from
Avion -Experimental car set world record for fuel
That being said, I could not find information on how the acceleration and top speed of the diesel versions. A 0.27 Cd is okay, but a lot of improvement could be made, allowing for substantial gains. If it had a 0.20 or lower with a reference area of under 16 ft^2, it would have absolutely no problem achieving 80+ mpg with the current, more stringent EPA cycle. And if it is the 3-cylinder diesel used in the Smart(it was specified that the engine was a 3-cyl Mercedes diesel, which I interpreted that to mean that this is an updated version of the car with a new engine 799 cc engine from a Smart; Mercedes didn't make 3-cylinder diesels in the 1980s for cars as far as I know), even the paltry 54 horsepower would still allow 0-60 mph in 15 seconds and a top speed in excess of 130 mph if the car had the gearing for it and the low drag proposed in this paragraph. That's acceptable performance, even if the acceleration might be a bit slow... a bit of turbo and injector pump work could double that and turn it into a rocket without harming efficiency.
In the event that it actually is a 240D engine, I would have doubts that it could achieve the fuel economy claimed on the recent record.