How does Spark plugs affect your Fuel consumption?

The spark plugs are the business end of the ignition system. They are an integral part of your vehicles motor as they provide the much needed spark that ignites the air and fuel mixture within the cylinders. This continuous ignition is what keeps your car moving on the road. No spark means no combustion, wasted energy, increased emissions, loss of performance, idle roughness, hesitation, hard starting and possibly even a no start if all of the plugs are affected. Spark plugs are normally a wear and tear kind of part instead of an "all or nothing" kind of failure. Over time, they get worn or build up deposits from years of use and they become less effective and efficient.  I have rarely ever heard of a spark plug failing 100% all of a sudden. As it wears out, it would contribute to reduced gas mileage (increased fuel consumption), rough idle, and sluggish performance. They recommend them to be replaced at a certain interval, every 30,000 miles, to keep top running performance. It is a very common part of a "tune up"

Spark plugs are designed to be self-cleaning up to a point. When the engine is running, the ceramic shell that surrounds the center electrode gets hot and helps to burn off any fuel or oil ash deposits that might otherwise foul the spark plug. The "heat range" of the spark plug determines its operating temperature and its built-in fouling resistance. The plugs should be hot enough to prevent fouling but not so hot that they increase the risk of pre-ignition and detonation. The spark plugs that are specified for your engine by the spark plug manufacturer should be correct for your vehicle application.

Full throttle acceleration and highway driving is good for spark plugs because it generates heat that helps keep the spark plugs clean. Short trip stop-and-go city driving and prolonged idling especially n hold ups for long hours, on the other hand, are NOT good for the plugs because the plugs may not get hot enough to burn off all of the deposits. So as deposits accumulate, the risk of misfire goes up. And if the plugs get dirty enough, they will misfire. The electrical energy from the ignition coil that normally creates the spark will short circuit to ground across the deposits on the center electrode instead of jumping across the electrode gap to ignite the air/fuel mixture.