I buy the quart bottle that treats 100 gallons, then put 1/5th of the bottle in every few tanks. I have tracked a consistent improvement in fuel economy with that tank every time. Probably not enough to cover the cost fully, but I have to assume it does good things for the engine then. I intend to follow the instructions and add to every tank in winter.

I became a big believer in fuel additives for engine cleaning with my older jag. Fuel injector cleaner seems to stave off a sporadic problem with one injector sticking in that car. So I now use fuel addiditives that clean engines in all my cars on occasion.