This is my ride, a Seat Mii, twin brother to the VW up!.
Fuel economy: 4,1l/100km
Curb weight (factory): 929 kg
As of writing this, we have only done 3 tanks in this car. So it is still in its "break-in" period, and is consistently improving its fuel economy. The latest tank had an average of 4,37l/100km (or 22,89 km/liter) but we expect to beat that on the current tank. We wont do any further mods until we are getting somewhat consistent FE figures.
Light weight wheels
Sealed some gaps on the front of the car
Philips EcoVision H4 headlights
The car will be lowered by cm front and back.
I will try to install a LiFePo4 starter battery, this should help reduce the weight of the car by 12-14 kg, and LiFePo4 is better at absorbing energy, which should make the regenerative braking (smart alternator) more efficient, thus reducing the load on the engine.
custom grill block
installing LED position lights, and generally switching to LED whereever this can be done legally (furthermore reducing alternator use)
If I could have my way, I would also swap all bearings for ceramic ballbearings, put on carbonceramic disc brakes and magnesium alloy wheels, but i'm guessing it would be both simpler and cheaper to have the car dragged about by trained unicorns...