Over 5 decades of driving & being interested in mechanic I've seen many (long forgotten) devices that will decrease your fuel consumption.

If I had fitted every single one then I would have to find a way of stopping my fuel tank overflowing every few miles !

Lets face it if any of these things worked the manufactures would fit them to make their vehicles more attractive to buy in this money conscious age.

I guy I knew a few years ago worked on diesel engine design - he was of the opinion that if they had not had to consider emissions control in the design todays diesel engines would consume 50% less fuel that they do today.