Prius is a heavy vehicle from the added generator, motor, plus engine and battery and don't forget the high speed 7.5% grade climb which will take HP to do. Going taller in the gearing is not always the answer and only helps if the engine generates enough torque and runs more efficiently at lower speeds and heavier loads.
As far as the Lupo . . . all those tweeks have been done and it only gets 78mpg!
The Lupo 3L was a special-edition made with the intent of being the world's first car in series production consuming as little as 3 litres of fuel per 100 kilometres (78 miles per US gallon or 94 miles per Imperial gallon). To achieve this the 3L was significantly changed from the standard Lupo to include:
1.2 litre 3-cylinder diesel engine with turbocharger and direct injection (61 hp, 140 Nm)
Use of light-weight aluminum and magnesium alloys for doors, hood, rear-hatch, seat frames, engine block, wheels, suspension system etc. to achieve a weight of only 830 kg (1830 lb)
Engine start/stop automatic to avoid long idling periods
Low rolling resistance tires
You need to look at each requirement individually. A diesel is going to get the best overall mileage with the least amount of trouble. 100 mph is not really a problem with a diesel, correct gearing and an aero platform. 0-60...add Nitrous or Propane for power runs only. An elongated insight with a 1.3L diesel/NOx/Propane would be a good bet.
You need something that builds torque per liter to move all that weight of the passengers and vehicle.
2006 Jeep Liberty CRD...Founder of L.O.S.T.
OME 2.25" Lift w/ Toyo Open Country HTs 235/75/16s
ASFIR Alum Eng/Tranny/Transfercase/Fuel Skids
2002 Air Box Mod...Air Tabs (5) on Roof...(3)each behind rear windows
Partial Grill Block with Custom Air Scoop and 3" Open Catback Exhaust
Lambretta UNO150cc 4 Stroke Scooter