I am using the Fuel Mizer now on my mini-van and it's great. There's no installation. You just stick it to the dash board where you can see it. It tells you when you are accelerating too fast or braking too hard, and curtailing these two activities as much as possible can help to improve your fuel efficiency.
It beeps at you and flashes lights to communicate your driving habits and that gets a little annoying. I probably won't drive with it on all the time but it's beneficial for retraining me. I'm more aware of my inefficient driving habits.
What the Experts Say
Car and Driver magazine tested the Fuel Mizer and reported a 10 percent increase in fuel economy.
One note: I used the fuel additive but I can't give you any kind of quantitative number for what mileage changes I experienced. From everything I've read, this kind of testing needs to be done in a very controlled environment and I would be a total hack if I tried to represent anything other than my qualitative experience with the products.
Here are some tips to improve your vehicle's efficiency without using gadgets or additives.
Replace your air filter.
Make sure your tires are inflated properly.
Remove ski racks or cargo carriers from the vehicle.
Finally, use cruise control and drive less than 60 miles an hour to conserve fuel.