From Wikipedia:
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)[citation needed]. To achieve this the 3L was significantly changed from the standard Lupo to include:
1.2 litre three-cylinder diesel engine with turbocharger and direct injection (61 hp, 140 Nm)
Use of light-weight aluminum and magnesium alloys for doors, bonnet (hood), rear-hatch, seat frames, engine block, wheels, suspension system etc. to achieve a weight of only 830 kg (1,830 lb)
Tiptronic gearbox
Engine start/stop automatic to avoid long idling periods
Low rolling resistance tires
Automated gearbox and clutch, to optimise fuel consumption, with a Tiptronic mode for the gearbox
Changed aerodynamics, so a \bold c_\mathrm w\, value of 0.29 was achieved
The 3L, along with the GTI and FSI, had a completely different steel body to other Lupos, using thinner but stronger steel sheet. The car had an automated electro-hydraulic manual transmission with a Tiptronic mode on the selector and an automated electro-hydraulic clutch. The car also had an ECO mode. When engaged it limited the power to 41 bhp (31 kW; 42 PS) (excluding kick down) and programmed the transmission to change up at the most economical point. ECO mode also activated the start/stop function, a feature that was new to European cars at the time. To restart, the driver simply takes his foot off the brake and presses the accelerator. In ECO mode, the clutch was disengaged when the accelerator pedal was released for maximum economy, so the car freewheels as much as possible, with the clutch re-engaging as soon as the accelerator pedal or brake pedal is touched. The 3L also has only 4 wheel bolts and alloy brake drums at the rear, along with many aluminum suspension components.
Initially, there were very few options on the 3L, as options added weight which affected fuel consumption. Those available initially were electrically heated and electrically controlled mirrors, fog lights and different paint colours. In order to increase sales, other options were offered including all-electric steering, electric windows and air conditioning. These options however, increased fuel consumption slightly. In 2001, a Japanese economy driver, Dr Miyano, used it to set a new world record for the most frugal circumnavigation of Britain in a standard diesel production car, with an average fuel economy figure of 119.48 mpg. In November 2003, Gerhard Plattner covered a distance of 2,910 miles through 20 European countries in a standard Lupo 3L TDI. He achieved his aim of completing this journey – which started in Oslo, Norway and finished in The Hague in The Netherlands – with just 100 euros worth of fuel. In fact, all he required was 90.94 euros, which corresponds to an average consumption of 2.78 litres per 100 km (101.6 mpg).
According to the Lupo 3L instruction manual, the 3L engine also runs on Rapeseed Methyl Ester (RME) without any changes to the engine.
During the period of series production of the Lupo 3L, Volkswagen also presented the 1L Concept, a prototype made with the objective of proving the capability of producing a roadworthy vehicle consuming only 1 litre of fuel per 100 kilometres (235 miles per US gallon).
The Lupo 3L shared its engine and special gearbox with the Audi A2 1.2 TDI 3L. As a result of this and other changes, this Audi A2 is also capable of reaching the same results as the Lupo 3L.
