Several years ago I added a magnet to the fuel line of a Geo Prism to get it to run smoother after everything else had failed. To my amazement it worked. The gas mileage improved too. Then I got the idea of using magnets removed from old hard drives. These magnets are many times more powerful. I put one on my Dodge Grand Caravan that had an on board computer that displays the gas mileage. Over about a year it climbed from 18 MPG to 23 MPG!
The government tested adding magnets to the fuel lines back in 1980. Two trucks showed a 6% to 9% improvement but some showed very little improvement. Back then gas was 30 cents a gallon so they concluded that it was not worth it. Today with gas running over $4 a gallon even a 6% improvement would be worth it. However if you use high power magnets and leave them on for a year or more you may well see a 25% to a 30% improvement!
This modification has a long history. Adding magnets to the fuel line has been used since World War 2. They were used on Mustangs planes to get them to run better on the cheap gas. They were used on tanks to get a longer range so they could outrun the enemy. When the enemy tanks ran out of fuel they were sitting ducks.
Here are pictures of the magnets added to the fuel lines of Dodge vans. The first is on a 2.4 liter engine and the second shows 2 magnets on a 3.3 liter engine.