This "Centurion" is from the same company that developed the xr3
XR3 Hybrid Personal Transit Vehicle: A 125 mpg Plug-In Hybrid Three Wheeler You Build From Plans
I would not believe the high mileage on the Centurion because of it's aero. However, given that it is the same company, I believe they use the Kubota D902 engine: 900cc N/A diesel, 25hp @3600rpm (redline), 41 lb-ft of torque @2600rpm (see attached BFSC map.
) They attatch this to a VW Type 1 transmission (up to ~'72 beetles/buses.) I believe that would be enough to get a 1000lb car (XR3 without hybrid system) to 125mpg with good aero given proper gearing and driving techniques. I am not so sure that I would believe their BS (nor buy their plans) but the logic is sound, the VW Polo Bluemotion has a curb weight of 2550 lbs and a coeficient of drag of .307 cd and gets around 70mpg with its 1.2?L engine.
The cool thing about using diesel tractor engines is that they are VERY durable/reliable, parts are relatively inexpensive even from the manufacturer due to how common the engines are, and they are very receptive to fuel conversions (veg oil, black diesel, propane, etc.)
As for the turbocharging comment, it is a double edged sword. A turbo will lessen the pumping losses and eliminate throttle plate losses because of boost pressure forcing air into the engine, especially in diesels which run much higher boost pressures than gasoline engines. However, like with anything if you USE the power that it gives, your mileage will suffer because the increase in air will cause in increase in fuel consumption to maintain stoichiometric ratio.