Just do a search in Google or Yahoo! for "increase mileage" and take your pick of all the devices and gizmos available. Or get several and produce your own fuel while you drive to pick up free money the government has for you now!

Seriously, the gasoline internal combustion engine by it's very design is only around 35% efficient. Diesels are marginally better due to the higher compression design and better energy density of the fuel. But if there was really a better, more efficient engine that could be mass produced you'd be driving it now. Why? Because whoever owned that technology would be the most wealthy and powerful outfit in the world. And people don't hide that kind of power.

Wouldn't you seriously consider running right down to the dealership and buying a vehicle that got 100MPG or more? Of course you would, and there's not a company around that doesn't know it. That's why it's so easy to sell any kind of magic device that can be made to sound reasonable, especially when the potential gains are very minor and therefore easier to quantify to potential marks.