do they improve milege and which work the best
None work. Your mileage will NOT vary.
if you install this, and drive slower, always drive more conservatively than you do now, and make sure your car is tuned up aned ensure your tires are inflated properly you WILL see an increase in mileage.
but if you dont install it, but still do all the other things, you will still see an increase in mileage.
so… the moral of the story is save your money for legitimate repairs and maintenance.