by 
The Curious Bag
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<h4 class="dechead">On One Hand: Plugs Degrade Over Time
Spark plugs are wear items. The material at the tip, where the spark ignites your car's air-fuel mixture, degrades over time. As it degrades, the spark intensity decreases. As that happens, the fuel ignition may not be as complete, so the car will add more fuel to compensate, increasing overall fuel consumption. By replacing your spark plugs at recommended intervals (as advised by the EPA), you can be sure that the spark stays "hot" enough for proper combustion.
On the Other: The Type of Plug Matters
Not all plugs are created equal. Certain spark plugs may even cause worse fuel economy or potential engine problems. Some don't create as strong a spark; others make big promises for performance or gas-saving improvements that just don't add up, according to the Federal Trade Commission.
Bottom Line
Follow your manufacturer's recommendations for both spark plug replacement interval and type of plug. Also follow recommendations for replacing spark plug wires and fuel and air filters. All of these components wear out--and as they do, they can all compromise your overall fuel efficiency.
Source:
EPA.gov: Tips to Save Gas and Improve Mileage (PDF)
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