What kind of truck are you driving?
Trailer tails are good for about a 5 - 6% fuel economy improvement on tractor / trailers. They should work on a trucks too. The fancier ones automatically open and close based on speed. Basic ones are about $2,000
Side skirts are more popular because they are cheaper for the same 5 - 6% improvement. They are about $800 (That is US testing - I don't know how the side protection required in the EU effects aero)
Wheel covers are the cheapest but lowest gains (1%) They cost about $300 for a set of 4.
Combined you can expect about 12% improvement.
Trailer Tail:
Side Skirt:
Wheel Covers:
