Ask LH: Should I Use Premium Unleaded Fuel In My Car?

Dear Lifehacker, My car recommends that I use premium unleaded fuel, but I'm curious as to how much I'd actually benefit — I've heard some people say it doesn't actually matter. Is it really worth the cost, or are they just recommending premium to get me to spend more? Sincerely, Confounded by Cars

Photo by TaxBrackets.orgvia Flickr.

Dear Confounded,

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.

Cheers Lifehacker

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Comments

    Fairly obvious point: even if it's not "recommended", give it a go, and see how your mileage responds. I get better fuel economy with the premium, and it's enough that I'm breaking even or even doing better by paying for the premium fuel. (And less fillups is always good, right?)

      Pretty hard to do that empirically when fuel prices fluctuate so much! #]

        Actually it's very easy...

        Always remember you're actually buying kilometres, not litres.

        Keep track of the Litres used per Kilometre for Regular for a few full tanks worth, then the same for Premium. Work out the percentage difference.

        If you pay attention you'll find Premium is always the same number of cents-per-litre more expensive, say 8c a litre more. (That's what it was when I last had an unleaded car.)

        With my previous car I got 25% more distance per litre, and it cost less than 10% more per litre for Unleaded fuel... So that means I was saving more than 15% - The break-even point for that car was around 70c-75c a Litre for regular, around that price they cost the same.

        At about $1.00 a litre I was saving 15% using Premium, and the percentage saving was higher when the prices went up because as a percentage the static price difference went down as the base-price went up.

        Then I switched to a Diesel car (a slightly larger vehicle), and I was saving about $70 a week in fuel costs when Diesel was about 15% expensive than unleaded.

        Last week my local servo was selling Diesel cheaper than regular unleaded...

        Yep. Ecky, you really are one of the great thinkers of our times, aren't you? FYI, you measure your fuel economy by the amount of fuel you use for a set number of kilometres (or how many kms you get per litre of fuel) and not by how much the price of fuel is at the bowser. It makes no difference to your car's fuel economy if the price of reg unleaded is $1 per litre, or $1.50. The only difference is how affordable it is for you to fill up. Signed...gutless

      +1

      My '08 Nissan Micra gets at least 10% better mileage with 95 RON "premium" than with 91 RON "regular". Since the price difference is usually less than 10%, I'm coming out ahead.

      If premium also happens to clean the engine even a tiny bit better than regular, that's an added bonus.

      Yeah, recently did a ~350km run - on PULP I did it with 1/4 of a tank left, with ULP on the way back, didn't quite complete the trip before the warning light came on.

    Ah, a US article.

    "Premium unleaded fuel, which usually has an octane level of 92 or 93 (compared with regular’s octane level of 87)"

    Our premium is 95 and 98. Regular being 91.

      Australia use RON, USA uses AKI which is (RON+MON)/2
      91 RON = ~87 AKI
      98 RON = ~93 AKI
      So their regular is the same as our regular and their premium is the same as our premium.

        Good to know, I was just about to do a quick google to see if something like that was the case.

        Up until today, I assumed those numbers were just a petrol-vs-ethanol percentage. Thanks to your comment and wikipedia I now know they get it by "running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane"

        I may be confused, but at least I know more than earlier. Thanks!

    Keep in mind though that changing from one to the other too often will cause problems, If you want to save money pick one and stick to it! #]

      What problems does frequent switching cause? I run both my cars on Shell E10 so only mix it up when I can't find a Shell. I would have assumed that by mixing 95 with 91 you would create a blended fuel (let's say 93 for arguments sake) and the engine would simply adapt to that. My understanding is that knock sensors are very dynamic and react within seconds so a change every few hundred kms would be well within their capability.
      However that's just assumption -- the last time I rebuilt the fuel system on a car it had carburetters and there wasn't a microchip to be seen so I'm happy to learn something new.

        The ECU takes a bit of time to shift the air/fuel mix, so mixing it up on an irregular schedule means it could be over- or under-burning. Consistency matters more than the particular fuel used.

    it would be nice if we could just buy unleaded fuel. not the 10% ethanol variety.

      It would be very nice... my daily drive (I live in Australia) is a 1959 Land Rover, not designed to run on E10. This means I have to use 95 octane AND put the additive in for cars designed to run on leaded fuel :(

      @kanthan - it comes down to state regulations. In NSW they decided that 92 RON fuel must be E10, but I found after moving down to Tassie that E10 at servos seems few and far between.

      @Will - My last car was from the leaded petrol era too. Regardless of ethanol content or not, I always ran it on 95 RON premium with a leaded fuel supplement. Leaded/Super when it still existed was rated at 95 RON, so I always figured best to continue that standard.

    If you put in Premium, your car will use less fuel and you will be able to drive longer with one tank.

      Care to elaborate?

      Funnily enough, my wife's '06 Holden Viva wagon doesn't seem to benefit at all from using premium instead of regular. The mileage is around the same.

      My point: horses for courses. "Your mileage may vary", if you will. ;)

    In oz premium is fixed at about 12c more than regular, my car gets better mileage on premium, as the percentage mileage boost stays the same (if i drive the same) there is a break even point at which it doesn't matter what i buy,when prices are above this i get premium, below regular. I used to work at a gas station and countless people commented when the price was high that they had to get regular because they couldn't afford premium at that price, i tried to explain how backward that was but they though i was an idiot shop attendant that didn't know what he was talking about. Also I had a guy with way too much money test his rally car with each type of fuel, one delivered a couple of percent more even though the same octane so probably a good idea to compare not only grades but brands, yeah it's a bit pedantic but just do it once over a month or two, it's not much effort and the savings will add up over the years.

    And for premium/regular diesel?

    I always get premium - Car runs a shedload better & I get around 50kms more a tank...

    Almost all newer cars, especially those with engines over about 2.2 litres will get better economy off premium which will make it more economical than regular unleaded. You also have a little more power. And it burns less, so causes less pollution. The choice is pretty obvious.

      Even engines below 2.2L can work a lot better on premium. My Alfa has a 1.4L engine that works better on premium than regular, the same is true of the two VWs in the garage that have a 1.4 and a 2.0.

      Most of the new high performance low capacity European engines work better on premium.

    Google your car make and model and see if it's equipped with a knock sensor, and if the ECU will advance the timing to take advantage of the higher octane fuel.

    My late model Mitsubishi Magna doesn't, but it runs rough on idle using 91 and smoothly with 95. General consensus among mechanics is that 91 is rubbish in Australia.

      What model Magna? I've got a TJ Advance and it's ran rough on idle since I had it. Did get better after a service once but still there. Might try 95 for that and in the hope of better economy.

        You might get a similar result by just asking your mechanic to lift the idle by a couple of hundred revs - and check your timing belt.

    If you have a sporty type car definitely use premium - the higher octane stops the more highly tuned cars from pinging/knocking/blowing up.

    I was hoping to see details of the supposed benefits of using premium in increased economy, that's what I'd base my decision on if my car could (which mine can) take both fuels, where's those details?

    Also with japanese performance import cars like skyline, silvias, Chasers ect.. You should always run those kind of car with Premium. In Japan they have a higher octane after our 'premium' which recomended for those type of cars.

      It rarely gets much higher than 97. 100-102 Octane is available in some places but it is incredibly scarce no matter where you are

    Did this excerise a few weeks ago with my swift. Premium was about 7% more expensive, mileage improved by 3%..result...not really worth it for my car...although I was using an ethanol blend for the base comparison.

      You would get a 3% improvement just by going to a non-ethanol "regular" unleaded, which is why there's a 3c discount for Ethanol. That said, it's getting increasingly harder to find non-Ethanol regular unleaded.

    Twin turbo V8 Audi...Its a must have or I get pinging and bad starting, and nowhere near as much power.

    In normal cars I've found some respond well with better economy and more power, but I think these are related. The more power your car makes, the less you are opening the throttle, therefor you should expect less fuel usage.

    I've never had good experience with Ethanol blends...all my cars have acted like there was a hole in the fuel tank and it just disappeared lol

    Another top refuelling tip told to me by my mechanic: don't wait until the fuel light comes on to refuel. Many modern cars have the fuel pump located in the fuel tank and the fuel itself is used to cool the pump. When the light comes on, the pump's only partially submerged and so not being cooled fully and therefore wearing out at a faster rate. When he told me how much the parts and labour were for a replacement, I realised it wasn't a small job so try and refuel when it runs low and have got out of the 'I can get another 40kms after the light comes on' mentality.

    I've got a 98 Honda CRV and an 01 Renault Clio. Both prefer 95 over 91, especially the Renault which mandates it (but did occasionally get 91 when our regional filling station ran out of the good stuff.
    At a guess, I'd say that since the Renault is designed for the European market and that 91 isn't (in my experience) sold there at all, that the engine wasn't designed with 91 in mind. The Honda on the other hand was designed to be sold globally and so 91-compatability would have been there from the start.
    Just an assumption...

    My Honda Civic Type R runs well on E10, and seems more sluggish on regular ULP. However, it positively flies on Premium. And on the odd occasions that I find the 101 octane Shell V-Power Plus (Do they still make it?) I swear I can hear the car chuckling to itself as it drinks that sweet rocket fuel!
    So the reality is that the preferred fuels for me are E10 (cheapest) or Premium (the dearest). There is usually 25c/litre and sometimes up to 45c/litre price difference between the two. Premium gets me about 10% better economy. So IRL I tend to use E10.

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