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. This one falls into the plausible-but-unlikely category. Writer Larry Edsall recently undertook a 5,000 mile road trip – one he does annually – 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 – and that 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, 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 – if any – improvement in mileage.
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