[Fiesta_Man69]

Paulo7 - in regards to the link - you will notice that the undertray pictured is extensive and covers most of the car: so its hard to compare to a little one that just slips under the front.

Also, be very aware of fuel saving claims made by manufacturers in general:

for example -
Holden's 3L V6 claimed use of 9.9 - even when deliberately chasing fuel economy figures most publications couldn't get below 11s; driving hard went into the 13s.
Even the Econetic - claimed 3.7 - no publication saw below 4, most mid 4s.
These are just two.
So therefore, % saving claims need to be treated with some scepticism.
Just a heads up.