I don't blame you for your confusion, what with all the petrol-based advertising and vague "recommendations" out there. You may, in fact, be well able to ditch premium unleaded fuel; unfortunately, the answer is a bit more complicated than a simple yes or no. So, we'll start by explaining how your engine works — without getting too technical — and what kind of fuel is right for your particular vehicle.
What Premium Fuel Does
Premium unleaded fuel, which usually has an octane level of 92 or 93 (compared with regular's octane level of 87), is designed to prevent knock on cars with high compression ratios. The compression ratio is the volume of space inside the cylinder, and if you use low-octane gas on a car with a high compression ratio, it's more likely that the fuel will ignite prematurely, causing your engine to knock and rattle (if you want a more in-depth view of how a car engine works, check out HowStuffWorks' article on the subject). A mild knocking isn't necessarily harmful to your car, but it certainly isn't something you want — and heavy knock could damage your engine.
What Cars Require Premium Fuel?
That said, the majority of cars are designed with a lower compression ratio, so that low octane fuel is more than adequate to power the engine without producing knock. In fact, few cars actually require premium fuel these days — usually it's just the luxury cars that do (though some luxury cars — like the Cadillac STS, for example — don't). The best way to find out what your car requires is to — surprise — look in the owner's manual!
If your car requires premium unleaded: Then you need premium unleaded. Your car has a high compression ratio and moving to a lower grade will likely cause knocking, which you really don't want.
If your car recommends premium unleaded: This is trickier. Years ago, says Edmunds.com, it was more of an all-or-nothing deal, but many cars today have internal knock sensors that change the game. Now, many cars can adjust the engine's timing to prevent knock, thus allowing you to use lower octane fuel if you so choose. If your car merely recommends premium unleaded, you can safely switch to a lower octane without risking engine damage. You will notice a small drop in performance — as in, it'll take you a half a second longer to accelerate from 0 to 60 — but since I'm not trying out for Fast and the Furious XCVIII, I've never bothered. Some argue that this level of precision could make it easier to merge on the freeway or avoid an accident, though, so it's up to you to decide whether the extra money is worth the extra half second.
If your car does not recommend premium unleaded: Then you don't need it. In fact, if your car doesn't say anything about premium unleaded usage, all you'll be doing is dumping more money into your car. Premium unleaded will do nothing to increase the performance of a car that wasn't designed for it, so don't waste your money. It may sound like you're giving your car the good stuff, but it doesn't notice the difference, and neither will you.
Doesn't Premium Fuel Clean My Engine?
Lots of companies have been advertising that their premium-brand petrol does a better job of cleaning engine gunk, and this is somewhat exaggerated. Some petrol makers (such as Shell) may put engine detergent in their fuel, but there's really no concrete data to prove that this does a better job of cleaning your engine. If you ever find you do have a problem with engine build-up, HowStuffWorks notes that you can just as easily buy detergent at an auto shop or petrol station and add it to the tank yourself.
So, in the end, the answer is pretty simple: read your owner's manual. You might be surprised at what you find, even if you have a luxury car — if it says recommended, you can go down a grade if you so choose, and if it says required, you should probably stick with the grade it recommends.
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