Over the years, countless companies have come up with devices claimed to reduce fuel consumption. The claims have ranged from plausible to the ridiculous. For example, wrapping magnets around your fuel line will not double your mileage and the only way you will ever get 200 mpg with a carburetor is to make it so small that air and fuel flow are so restricted that it can't possibly use more fuel than that. Of course your engine would produce so little power in the process as to be useless.
Spark plugs have long been a popular area for "innovation" of this sort. One of the latest comes in the form of the E3 spark plug featuring its DiamondFire electrode. This one falls into the plausible-but-unlikely category. Writer Larry Edsall recently undertook the latest iteration of a 5,000 mile annual road trip and decided to try out the E3 plugs. While E3 claims a 3-5 percent improvement in mileage, Edsall found his dipped from 20.11/19.98 the previous two years to 19.18 with the E3 plugs. He did all the usual maintenance like oil changes and tire pressures before leaving. Of course the vehicle was a year older and more worn which may have contributed to the dip. But even a fully scientific test with two or more vehicles running side by side in the same conditions (aside from the spark plugs) would probably yield a negligible difference at best. The E3 plugs aren't that pricey ($6-7 each) so if you do choose to try them it won't cost a lot. Just don't be too disappointed if you don't see much improvement in mileage.
[Source: Detroit News]