agree with takuya, i have installed so-called micro-compressor, air regulator, fuel magnet on the second day i got my new car few years back....i feel the fuel comsuption almost same as other same model car.....minimal effect maybe, because i have 2 sub with box at my bonnet....hehethose fuel saver will have very minimal effect. do you know how a fuel saver is tested? they drive the car with the device very carefully and of course try to minimal the speed as possible, feet also light as possible, avoid all traffic if possibe..avoid any turn if possible, use straight road if possible, less gear change as possible..
So..
If you manage to do all the above, please buy the device to get a very small small saving on fuel..
Otherwise if you not able to do so...you understand...
best fuel saver is ur leg