[elhigh]

Your english is better than a lot of people I've known, so no worries.

The Yaris is pretty light and if you're mostly using it for longer trips, there's probably not a lot of point in any lightweighting. Aerodynamic improvements like you describe are probably going to be your best bet.

Don't forget about wheel skirts. Anything to reduce aero load will reduce the engine load. The belly pan is subtle and people won't notice it, but I think Europeans will give you less hassle than Americans might, if you decide to do something more visible.

Evidently flex fuel and E85 mods for your car have been done, I just read some entries in a couple of forums; those were on a quest for power but I reckon you could tune for efficiency. The parts and software won't be intended for it but I don't see why they couldn't be pointed toward minimum consumption instead of maximum output. They're just opposite sides of the same coin.