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Choosing the best spark plugs will depend heavily on what type of vehicle you own. Newer vehicles may require spark plugs with two or more ground electrodes, or ones that contain rare elements, like iridium or platinum, that allow for very high temperature operation. Older engines may benefit from a spark plug line that has a range of plugs to fit the engine, some of which burn hotter than others. If an engine is using a little oil and the old plugs have deposits formed on them, the best spark plugs for the application might be a few heat ranges higher than were originally installed.
Newer engines often come from the factory with plugs that include multiple-ground electrodes or special metals. These are typically designed to last through much higher mileages than traditional spark plugs. If and when they do need to be replaced, the best spark plugs to use will usually be ones that match the original equipment (OE) specifications. In these applications, the electrodes will be composed of, or coated with, metals that help fight against wear. These plugs will often last upwards of 30,000 miles (48,000 km), while the older, copper-cored nickel electrode plugs typically need to be replaced at around 15,000 miles (24,000 km).
Many spark plug manufacturers offer platinum and multiple-ground electrode plugs for vehicles that didn't originally come with them. Whether these are the best spark plugs for your car may depend your budget and your needs. Such plugs often cost two or three times more than traditional plugs, and it is questionable whether they offer any benefits other than lasting longer. If you own a vehicle that has plugs that are very difficult to reach, it may make sense to save labor by installing longer lasting plugs.
If you have an older vehicle that burns a little oil, higher priced platinum units may not be the best spark plugs for your vehicle. Choosing a traditional nickel alloy plug a few heat ranges higher than OE may help prevent oil deposits from forming. In these applications, platinum or multiple-ground electrode spark plugs will tend to simply foul out and require replacement. This can create added costs, both in labor and the fact that the plugs themselves are more expensive.
Your owner's manual will usually indicate which type of spark plugs were originally installed. Failing this, a dealer may be able to provide the information. Auto parts stores often have catalogs that indicate which type of plug was originally installed. If a spark plug line offers multiple heat ranges, the catalog may also have a conversion chart to determine which plugs from a higher heat range may be substituted in a given vehicle.