Quote:
Originally Posted by cRiPpLe_rOoStEr
Slightly narrower tyres can improve the mileage, altough it leads to some worse braking performance.
Even spoilers, sideskirts and ailerons? Ironically, one of the most praised aerodynamics specialist was Prof. Wunibald Kamm, a German national...
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If you change anything, spoilers, body kits, wheels, lights, air filters or whatever, the part needs a test certificate to be used with that car.
Commercial spoilers or body kits or alloy wheels come with such a certificate (called ABE). Those are common in the custom car community but not designed for economy.
Home projects are more complicated: You have to show up with your car at a local technical specialist and ask for an expert testimony. Then you have to visit your local registration and add the change to your car's papers. This is more expensive than any savings you might have. (at least several hundred Euros). And many mods will not pass the strict rules at all.