What kind of additives are you guys talking about? I'm curious too, I'd love to be able to economize more, wherever I can. I already stopped taking the car to the car wash, I just wash it myself most of the time. Whatever helps to reduce monthly costs.
I tried one and I did not notice the difference in gas mileage. I don't really know what the additives are supposed to do. I do use dry gas each winter though. That is supposed to keep the moisture out of the gas.