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Is there any evidence that premium and/or high-octane fuel (petrol/gas for cars):

  1. improves performance; and/or
  2. improves engine longevity; and/or
  3. improves mileage so that money is saved on fuel, even when taking into account the extra cost of it

I'm somewhat sceptical of the claims of fuel companies, especially given the price of premium fuel. Have independent tests been performed?

Thomas O
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    I was under the impression that higher octane fuel only suppresses combustion, so you can run at a higher compression ratio (and thus get more energy from the same fuel) without causing pre-ignition (engine knocking). If an engine runs on low-octane fuel without knocking, I would be skeptical that it would run any better on high-octane fuel. – Mike Dunlavey May 18 '11 at 22:29
  • @Mike Dunlavey: modern ECU's have variable ignition timing and fuel injection; they can detect knocking and compensate for it. That's why they can run better on high-octane fuels; it allows the ECU to inject more fuel and ignite it later (when it's distributed better throughout the cylinder) – MSalters May 19 '11 at 12:48
  • I'll have to dig up the reference, but I recall seeing a test demonstrating that it's engine-related. High-octane fuels were beneficial to engines with a high power-to-displacement ratio. The Focus RS mentioned by Ardesco is such an engine, a 2.5L turbocharged 5 cylinder engine. – MSalters May 19 '11 at 13:00
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    @MSalters: Ummm... wait a minute. You don't save fuel by turbocharging, or by injecting more fuel. To prevent knocking you have to inject the fuel into the cylinder (as in a diesel), not spark it later. High power-to-displacement engines get that way by having a higher compression ratio, thus need higher octane, regardless of turbocharging. High volume ratio provides more power by making the pressure volume curve larger, for the same chemical energy. – Mike Dunlavey May 19 '11 at 18:03
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    @Mike Dunlavey: turbocharging etc does not directly cause a performance increase with premium fuel. Instead, it's the underlying design. When your design goals are to maximize power per volume, you end up with both a turbocharger and a high compression rate. The latter in turn requires premium fuels. So there's indeed a correlation. – MSalters May 20 '11 at 08:49
  • Depends on the car. Higher octane fuel actually contains slightly less energy. It has a smoother burn, so will work in conditions where lower octanes will detonate (or even pre-ignite, but that's pretty rare). Unless a car states that it is designed for premium fuel, the ECU maps will max out at values designed for regular and using premium instead will be of no benefit. Indeed, you may experience a very tiny drop in MPG as well. However, if the car *is* mapped for premium, it will perform better (despite less energy) due to the mapping being able to take advantage of the higher octane. – Brian Knoblauch Jul 01 '11 at 13:56
  • My mum's garage (which also sells fuel) told her that cheap, fuel e.g. sold outside super-markets contains fewer additives which help to keep the engine clean and reduce maintenance costs (my mum drives a diesel): so, not about power but about longetivity. – ChrisW Jul 01 '11 at 15:17
  • My father, in the 60's, put aviation fuel (about 128 octane at the time) into his V8 Opel Kapitan. It was fundamentally quicker for about 30 kms until it, the engine, spread itself over the E30 as it is known now. Higher octance fuel does increase power, and increases the effciency of the burn. I am not too sure of the financial benefits of paying 8p a litre more in some cases in order to gain around 0.8th mile per km. [better power and consumption figures with higher octane fuel...](http://www.thorneymotorsport.co.uk/tuning/ford/focus-rs/focus-rs-fuel-testing.shtml) – Hairy May 19 '11 at 13:46
  • Company I worked for changed their policies for fuel reimbursement because after several years of collecting data it became clear there was no real difference in fuel economy between "premium" and "regular" gas, this was done based on calculating the real average mpg of hundreds of cars we have on the road, part of which used premium and part of which regular fuel. (of course not open to the public, it's on our intranet only) Testing it myself (driving 5000km on premium only, then 5000km on regular only in the same car after having it serviced) yielded the same conclusion. Of course we didn't – jwenting May 19 '11 at 07:18
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    See http://mechanics.stackexchange.com/questions/39/what-are-the-benefits-of-premium-high-octane-petrol – endolith Jul 09 '11 at 16:04
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    I've never heard fuel companies claim that premium fuel is good for cars. They often claim that THEIR fuel has additives that are better for your engine, but don't make a distinction between the grades. – JohnFx Jul 10 '11 at 01:41
  • @endolith nice link. If you don't mind I'll add it to my answer which says the same thing, so maybe people will quit downvoting it. – psusi Jul 11 '11 at 13:53

3 Answers3

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Seems to be a yes based on this:

http://www.thorneymotorsport.co.uk/tuning/ford/focus-rs/focus-rs-fuel-testing.shtml

Remember this is completely standard car, no mapping no toys just as it came from the dealer. Peak power went from 272bhp to 293bhp – 20bhp for doing nothing more than putting better fuel in the car. MPG also went from an average (dash display so hardly that accurate) 22.3mpg to 24.8mpg and that only took three tanks, its now on 25.6mpg now that its modified. Its a free lunch, it really is. But don’t trust us, try it, just run three tanks through the car, we can guarantee that the car will feel better, you’ll get more power and cover more miles, try it.

Fuel Comparison

Ardesco
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  • ThorneyMotorsports worked on my car, transforming it from 217bhp to 370bhp, with the parts I suplied. However, these people replaced 3 abs sensors, charing me for them each time, until they realised the ECU was firing up failed ABS warnings as too much power/Air was going through the MAF sensor, and the ABS sensor warning was the first one on the 'get the car into limp mode' list, so it simply fired that, rather than create a new mode for jesus H, how much air are you trying to force through me... In short, I am not 100% convinced Thorney know much about what they are doing. – Hairy May 19 '11 at 13:42
  • I would, however, add that they seem to be on the boil re fuelling a car; they advised me to address fuelling first on any high power conversion. Caveat emptor I believe is the term. – Hairy May 19 '11 at 13:43
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    In the USA, "Premium Fuel" is usually 92 or 93 Octane, with "regular" being around 87 Octane. (That's one reason why fuel is relatively cheaper in the US, and also a reason why some fuel-sensitive sports cars aren't marketed here.) I wonder if the difference between 93 *vs.* 87 is as pronounced as 99 *vs.* 95? – ESultanik May 19 '11 at 13:46
  • @ESultanik there would be as measurable a difference as there is over here. It is better for the car, for the engine, as it is designed for it. However, most cars, in europe, are 'mapped' to take into account the variability of the fuel qualify across europe. Just mapping your car, correctly, to run on ordinary fuel (95oct), will see up to a 30% increase in power and a marked improvement in fuel consumption. If you look at the costs involved (250-400), and then look at 52 fill ups a year, using 95 over 98 and having a map is much better (financially and performance wise), over a year or so. – Hairy May 19 '11 at 13:59
  • @Hairy I was just wondering how big that measurable difference would be. Also, I speculate that the difference is also a function of the size/output of the engine (the more powerful the engine, the more the potential gain from better fuel). I'm also aware of re-mapping; I've a re-mapped US-Spec Rev. B Subaru WRX that recently dyno'd at 320bhp (on 93 Octane) :-) – ESultanik May 19 '11 at 14:22
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    Note that US 91 = UK 95 octane. UK uses RON number, US uses AKI. – horatio May 19 '11 at 14:52
  • I'd say Thorney have measured it, at about 7%. No mean feat, but I'd imagine you could improve a Forced induction car by 10% by simply putting on a better intercooler and better induction. I raised a 210 bhp car to 235 doing just that, without mapping or anything. I'd argue that the size/output of the car would govern the improvement. James Turvey of Superchips UK told me that there is always a capacity in todays car to improve the performance of the engine by around 10% without re-mapping; todays ECU's are that intelligent, they can adjust to the better fuel (EU notes here). – Hairy May 20 '11 at 06:28
  • @horatio: Wow, I didn't know that. That pretty much renders my question moot! Thanks! – ESultanik May 21 '11 at 12:03
  • so US "high octane" fuel is European regular... – jwenting May 23 '11 at 11:30
  • Also, it is obvious but easily overlooked by US readers that if/when a UK site quotes MPG, a gallon is not the same size. – horatio May 24 '11 at 13:58
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    1 UK Gallon = 4.55 Litres 1 US Gallon = 3.79 Litres (Obviously approx) Also our Pints are bigger than US pints so anybody coming over here and having a few pints, beware you may be drinking more than you expect :) – Ardesco May 24 '11 at 15:52
  • Oh that also puts UK fuel prices at an equivalent of £5.17($8.37)/US Gallon for regular and £5.45($8.82)/US Gallon for premium. Of course prices and exchange rates are in flux so this should be treated as approx figures that are only correct at the time of writing. – Ardesco May 24 '11 at 15:59
  • I don't believe that someone with a *vested interest* is a very *reliable* source. Also there is a *huge* difference between a performance car and a run-of-the mill car that most people use. – Mark Booth Sep 27 '11 at 10:39
  • Not sure I get your point, a vested interest in what? How would encouraging people to buy higher octance fuel benefit a tuning company really? They don't sell the fuel, they won't gain anything from people buying it... – Ardesco Oct 13 '11 at 20:30
  • And I agree there is a difference between a performance car and a run of the mill car, but there is also a huge difference between every car. The OP asked if there was any evidence premium fuel can result in better performance, the above is evidence. – Ardesco Oct 13 '11 at 20:33
  • A single data point (i.e. one dyno graph) doesn't tell the whole story. A significant issue in how high octane fuels perform is the altitude. At higher altitudes there may be no advantage to higher octane fuel: http://www.dmperf.com/fuel_brands_and_octane_ratings.htm "An engine that might require 93 octane at sea level may perform at maximum on a fuel rated at 91 octane if the elevation is over, say, 1000 feet." – Dan Haynes Apr 08 '12 at 16:59
  • The link is dead. And to add anecdotal data. I am driving a 99 Chevy Prizm daily about 50mi (with roughly 2/3 highway at sea level in central Florida). I consistenly get 29-31mpg with 89, 26-28mpg with 87 and around 32mpg on 93. – Stefan Nov 01 '12 at 17:15
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    Even if their numbers are accurate ( could be for a specific engine ), and generally applicable ( not going to be ), it is no "free lunch" since the higher octane fuel costs considerably more. – psusi Dec 09 '14 at 04:19
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"Car and Driver" did a test which concluded that cars designed for premium fuel (which in general are higher-performance cars) do run better on premium fuel.

So, at least one of the claims can be confirmed. With respect to the longevity claim, Car and Driver offers an opinion: a dirty engine will increase knock, which can be ignored by using premium fuels or solved by cleaning the engine. The reason is that soot deposits act as ignition sources.

MSalters
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  • +1 Right. Cars with higher compression ratios need higher octane fuel. I had the impression the question was "All things being equal, is it better to use premium fuel?". Like my jalopy runs fine on 87, & I doubt it would run any better on 93. But, if the only change to the car was to get the same power from a smaller displacement engine running 93, would it save enough gallons to pay the extra price per gallon? I guess I don't know. But I agree with what you say about running clean. – Mike Dunlavey May 19 '11 at 18:22
  • Higher compression engines run hotter, which does improve the thenoretical efficiency limit (Carnot efficiency), and also practical efficiency (no surprise, given the importance of engines to high-performance cars). But monetary efficiency? High-performance cars are generally owned by people who are less price-sensitive, and AFAICT premium fuel prices reflect that. – MSalters May 20 '11 at 08:42
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No. Higher octane fuel is not "better", it simply burns at a higher temperature/pressure. The advantage that high octane fuel has is that you can build the engine to use a higher compression ratio which allows you to get more fuel, and hence, more power, into and out of each stroke. If the fuel is too low octane for the engine, you will get pre ignition ping, otherwise known as knocking, because the fuel explodes too early. If the octane is too high, then it explodes too late or won't burn at all. These days the electronic control systems can play games with the valve timings to compensate somewhat so that you don't get knocking, but it still isn't doing the engine any favors to use the wrong fuel.

So the bottom line is: use whatever fuel the engine was designed for, no higher, and no lower. For most non sports cars, that means 87 octane.

See:

http://en.wikipedia.org/wiki/Engine_knock

http://ask.metafilter.com/128168/Does-high-octane-gas-matter

http://www.toyotaperformance.com/fuel_octane_vs_horsepower.htm

https://mechanics.stackexchange.com/questions/39/what-are-the-benefits-of-premium-high-octane-petrol

There also is, of course, the owners manual of pretty much any car that will tell you what octane fuel to use, and they don't specify X or better. Some manuals, like my Toyota Prius, explicitly specify NOT to use higher octane fuel.

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psusi
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  • **We expect users [to back up any significant claim](http://meta.skeptics.stackexchange.com/questions/5/must-answers-be-referenced/343#343) with a reliable source**. Please edit your answer to include the references on which you based your answer. – Oddthinking Jul 01 '11 at 08:21
  • @Oddthinking fine, there's a few – psusi Jul 01 '11 at 14:17
  • Hopefully with your edits, some people will retract their downvotes, this is now by far the best answer here, though it would be even better if someone could find some truly independent, ideally scientific, studies of the issue. – Mark Booth Sep 27 '11 at 10:41
  • Interestingly, mine does say "X or better." -- My Mazda 3 (not speed edition) says: Regular unleaded fuel 87 [ (R+M)/2 method] or above (91 RON or above) – ckittel Dec 09 '14 at 01:33