Good article. Car makers use things that actually save fuel, such as start stop systems, even if they are expensive. Regenerative breaking is used too, charging the battery and thus saving fuel. So if these simple devices would work they would be used.

Those that were professionally tested all failed and increased (!) fuel consumption. So unless there is one that proved it works in proper independent testing I think it is safe to say they are all scams.

I believe in all the videos the savings were negative. Without device 49 mpg, with 47 to 48 mpg. So fewer miles per gallon. They HURT the fuel consumption.