The most common products that claim to add some extra MPGs onto your drive are the pour-in-your-gas-tank additives. You'll see almost an entire shelf devoted to them at the automotive parts store and most of them have two basic claims. The first is that they clean out parts of your engine. The second claim is that they increase or restore gas mileage because of or in addition to, the first claim.
Of course, whether or not the additives truly help clean out parts of your engine would be difficult for most vehicle owners to actually prove. Some of the areas these additives claim to clean are not easily accessible to the person buying them and therefore hard to properly evaluate on a case-by-case basis. Or, if they do clean the engine, the change is so insignificant that it has no effect on the engine's mileage.
Some people put these additives into their vehicles at every fill-up and others may only do it occasionally, but either way you're most likely not getting what you've paid for. Even if the additives do clean areas of your engine like they say, according to the EPA's tests there are no additives you can put in your car that will increase gas mileage. The EPA tested 14 different fuel additives and none of them were proven to have any positive effect on a vehicle's gas mileage. You'd be better off saving the few dollars you'd spend on the additives and actually buying a gallon of gas with them.
But additives are just one category of products that claim to increase your gas mileage. Take a look at some more intricate systems on the next page.