Got the sidepieces installed today. I decided not to turn them into air deflectors for the wheels, and install a smaller flimsier bit for that later instead. (having low ground clearence and all that)
Plenty of work left it seems:
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2016: 128.75L for 1875.00km => 6.87L/100km (34.3MPG US)
2017: 209.14L for 4244.00km => 4.93L/100km (47.7MPG US)