[Piwoslaw]

Smooth, driving style is the most important thing you can do, but I'll focus on the car mods I think you'll get away with:

• Inflate tires to to max sidewall pressure, or max minus 5-10% if you are not comfortable with max yet. TÜV doesn't check tire pressure.
• Grille block. There are some nice looking grille covers available for many models. Here is one for the Skoda Fabia. Of course, you can make your own, and even make it openable, or start with something more stealth.
• Engine kill switch. Not that I'm soliciting engine off coasting, first you should make pulse and glide (with engine on) your second nature. The engine off button would be used at red lights. But once you get the basic and intermediate hypermiling skills covered, you'll find that in many cases engine off coasting and bump starting will save quite a bit of fuel without sacrificing safety too much.
• Alternator kill switch. Can save fuel by as much as 10%, you can even try to automate it, as is done in many new BMWs, Skodas, etc.
• Mini-Kammback. My opinion is that if your Kammy is stable and won't fly off, but 100% removable and doesn't stick out beyond your bumper, then no one should give you any trouble. But first you can try a shorter version. I've seen mini-kammback-oids available as tuning parts for Opel Astras and Corsas. Nobody even notices them, but by fine-tuning the angles you will see an FE improvement. If you make one that looks half-decent, then no one will care.
  
  
  
  
  
• Flat hubcaps. Vekke is starting an ecomodding company and I hope that among his first products will be flat plastic hubcaps. Keep an eye out for them.
• Bellypan. Real gains and invisible, except at the mechanic's garage, and maybe when entering/leaving the EU.
  
• Block/coolant heater. This is a heating element that is installed in your engine or in its coolant line. Other than having almost instant warm air in the winter, more importantly you get lower fuel consumption, lower emissions, and lower engine wear. I'm sure you'll have less trouble finding a DEFA heater than I did.