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In general, higher octane gas increases the cost of the fuel, but offers no real benefit to most vehicles versus its lower-priced counterpart. Despite this, there may be situations where higher octane gas might make a difference. Determining what is needed for your specific car is done by looking at the manufacturer's recommendation and listening to your car's engine.
Higher octane gas is desirable by some because they feel it keeps their car engines running smoother and is a cleaner fuel. While the latter part may be the truth, most car engines will notice very little difference with the higher grade gas. The cost-benefit analysis is also a loser for the higher octane gas. In many cases, filling up with higher octane will only increase the amount of money coming out of your pocket.
Your vehicle's recommended octane is noted in the owner's manual. If you bought a used car that did not have a manual, or if you lost the manual, consult the manufacturer's Web site. Many have the manuals or at least certain specifications about their vehicles, in an online format. Generally, it is not necessary to put in a higher octane gas above the manufacturer's recommendation. It should also be noted that an octane below the recommended should not be used either.
For the vast majority of vehicles out there, 87 octane will be the recommended grade. However, there may be some performance vehicles that do require a higher octane. These automobiles are generally not ones most in the market are interested in. Still, there could be situations where a family vehicle needs a higher octane gas.
Listening to how your car is running is another key to determine if you may need a higher octane gas. In most cases, the engine should sound very smooth and consistent when it is running. If you hear a knocking sound, that could be an indication you need a higher octane fuel. Trying such a solution the next time you fill up may yield good results. It should be noted this only applies to gasoline engines. Diesel engines, by their very nature, produce a knocking sound.
In some places in the world, such as the Midwestern United States, there may be situations where a higher octane gas costs less than the lowest octane. In the Midwest, this is because of the use of ethanol and the abundance of it in the local area. Many may choose the higher octane due to the lower price. Higher octane gas will not hurt an engine so for those who wish to do this there are generally no problems.
Years ago, in the era of leaded gasoline, infant unleaded and stinky catalytic converters, my dad advocated the occasional tank of "hi-test" or premium gas. This did help clean out a carburetor and would often help a car run better. He said it would flush trash out of the fuel line. Perhaps there was something to that. He got 260,000 miles from his 1977 Chevy Nova before it finally died on him.
Nowadays, with fuel injectors, I don't know that messing with the octane level is a good idea. Computers control the cars and are programmed to perform with a particular octane level. I have heard of using a bottle of injector cleaner every so often, to get the "gunk" off the fuel injectors, but I think mechanics are a little leery of advising customers to use an octane level other than what the car is rated for, except in an emergency.
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