The only product I have used that works is Lube Control FP Plus.
It seems based on my gas logs that it gets me about 1 to 1.5 mpg.
The two bottom tanks I forgot to add the FP Plus
The high tank had about 40% of highway on it.
Usually I only have about 10% highway on each tank.
Once I graphed it out it proved to me that the gas additive did work.