Hi i have been driving a lupo 3l for around 20.000Km and i recently bought an Audi A2 1.2tdi.(bought it 3 months ago but crash it first day)
I can manage 1.9~2.1 l/100 Km but it mostly depends on the road traffic and specially driving speed. it is very important to measure the altitude difference in your records as going uphill or downhill makes a lot of difference.
My car is stock (i didn't do any modification).
I would say the easier way to get under 2.0l/100 would be 60~80Km/h speed and in a road with ups and downs.(not highway) My advise is never let the car disengage the 5gear on uphill and glide until around 60Km/h. (its a patience game) and try to never touch the brake pedal.
Did you ever think on using biodiesel? I think the bigger battle will be the €/100Km and its probably the best economic viable way to drop the money consumption on the lupo.(and protect the environment) And the lupo is already adapted to use it.
The audi A2 is better on fuel consumption than the lupo and i think one of the reasons is the thinner tyres. The A2 uses 145 taller tyres instead of the 155 so i think that the smaller they get the better. Another feature the Audi A2 have is the smaller fuel tank (just 20lt) to the lupo is a 13Kg difference. For the wheels the A2 caps are great and you can buy them easily on AUDI and they fit the lupo (same rims). The oil on the engine makes a big difference in consumption so i would not put anything but the recommended oil(or better).
Did you already checked the alignments of direction? fit it to 0º. Another thing i would recommend is use tape to seal the gaps in the body panels.
I would also fit front well wind deflectors and increase the rear ones. Lower suspensions is also pretty simple and inexpensive.
For harder modifications i would first think on installing an electric engine (hybrid lupo
) before chop the lupo (cheaper and maybe more fuel efficient)