Your car’s spark plugs ignite fuel by means of a spark. Misfiring spark plugs can reduce fuel efficiency by as much as 30 percent, or 75 cents per gallon. Even worn-out spark plugs can lead to poor engine performance and lowered fuel economy. Replacing them improves combustion and reduces emissions. Refer to your vehicle owner’s manual and follow the manufacturer’s recommendations on how often to replace your vehicle’s spark plugs.