Fuel saving devices are a gimmick. Don't lose your money on them.
There is PDF file from US EPA that has tests results for over 100 such devices, most of which don't work or even reduce the fuel efficiency. No device could produce better than 6% fuel efficiency, which is not worth the cost.
I had some info that there are devices that can increase the fuel efficiency of 2-stroke, single cylinder bikes to over 10%. But that's it.