yeah they allow the fuel/air mixture to burn more efficiently
Brands such as NGK, and Bosh both make plugs that are said to help gas mileage.
If they are dirty, yes.
Yes.Yes.
If it needs new spark plugs then that will help gas mileage but if the plugs are firing ok and gapped correctly then new plugs will not help. Adding true dual exhausts will add a little power and may help mileage slightly but you will prpbably never ever recoup the cost of the dual exhaust system in fuel savings.
If they were real dirty, yes.
bad gas mileage, hard starting, and basically bad performance.
NGK makes fantastic plugs that produce excellent spark for your engine.
As the plugs wear, the gap increases and the spark fades, resulting it poor ignition eventually. This simply means reduced gas mileage. I have 60000 miles on my stock plugs with no reduction in gas mileage, but I plan on changing them real soon...
New spark plugs with the proper gap will give your car the best possible gas milage.
The gas mileage on a 2001 Millennia S can be improved by performing a tune-up. This includes replacing the fuel filter, air filter, spark plugs, and cleaning the injectors.
Your Ford Focus may be getting poor gas mileage due to a clogged air filter or worn spark plugs. A complete engine tune-up can often fix most issues with poor mileage.
There is a numerous number of reasons. Starting from piston rings and spark plugs and ending by tires, quality of gas and engine oil...