The auto parts stores are full of them: racks of cans, bottles and cartons containing additives that claim to enhance power, reduce wear and improve engine performance -- and all you have to do is pour them into the oil pan or fuel tank. Thanks to their relatively low cost, these are some of the most often-seen mileage-enhancement devices. But not all of them work as advertised.
Modern car and truck engines are the result of decades -- even centuries -- of refinement. Parts do wear out, and any device that turns a series of small explosions into propulsion is bound to wear down over time. But vehicle engineers test engines for tens of thousands of hours, often under conditions worse than a road-going car would ever encounter, and design their products accordingly. As a result, even fuel and oil additives that work as claimed may only show minimal benefits when used.
Some oil additives, for example, are advertised with a striking commercial: an engine is run with the additive in it, and is then drained of oil and restarted. Miraculously, the oil-less engine runs. What the advertisement doesn't say is that the engine, like many modern engines, is likely sturdy enough that it can run temporarily with no oil in the pan. The running engine is less a tribute to the oil additive than to the engineers who designed its tight tolerances and high-wear materials [source: Tony's Guide to Fuel Saving Gadgets].
Some additives may indeed work. But understanding which ones can make a difference for your car or truck requires a thorough understanding of both the engine and the additive.