And this one has been proven by taking it on a very long journey...

Based on the Triumph Tiger, the engine is an 800cc lump from a Smart car. Another owner reckoned it was returning 120mpg and happily cruised all day at 80.
Specs:
Engine: Inline 3 cylinder, liquid cooled, turbocharged diesel.
Capacity: 799cc
Fuel system: Common rail, electronic with pre-injection.
Power: 45 ps (approx without further tuning)
Torque: 100nm (approx)
Top speed: 100mph / 160km/h
Fuel Consumption: 100mpg (2.77 l/100km) real world with full luggage, 120mpg (2.4 l/100km) ideal conditions
Transmission: 6 speed, chain final drive.
Fuel capacity: 24 litres
Front Suspension: 43mm forks
Rear Suspension: Monoshock adjustable preload
Front Brakes: 2x 276mm discs 2 piston sliding calipers
Rear Brakes: Single 255mm disc 2 piston sliding caliper
Front Tyre: 110/80-19
Rear Tyre: 140/80-17
The price is under £10K sterling, so with the Us's smaller taxes etc it shouldn't be hugely expensive to buy and ship one over-no more than a standard Tiger.
Seems to be a succesful machine and it's at the right sort of price.
Website here:
http://www.suckindiesel.com/