By on June 23, 2008

340x.jpgA guy says he’s stopped using premium gas in his “premium gas required” car because it’s too damn expensive. It’s a joke, right? He’s saving 30 cents now, only to threaten his warranty and pay thousands in repairs later? “Yes” is the easy answer. But the truth about cars can be a funny thing, especially when you add fuel and flames.

With apologies to the chemists, theoretical physicists and tuners out there, here is an octane apercu: octane rating measures knock resistance. It has nothing to do with energy content. Engine knock (or ping) occurs when fuel detonates before the piston is in the right spot. The temperature and pressure in the cylinder cause the fuel-air mixture to detonate prior to the spark.

The effect is like tapping the cylinder head with a ball peen hammer, hence the sound. Add the intended spark, and you can have two flame fronts in the cylinder smashing together in the kind of closed-cage match no one leaves. Seriously, knock would destroy a lot of engines if not for octane.

Octane (a.k.a. 2,2,4 trimethylpentane) is a chunk of carbon and hydrogen added to gasoline to prevent premature detonation. In a bit of counter-intuitive chemistry making phrases like “high octane thriller” slightly ridiculous, it actually slows the combustion process. Octane’s fame as a performance enhancement comes from allowing higher compression, which results in more punch. The higher compression comes from the factory, not the pump.

The octane rating itself comes from good old fashioned testing. And I do not use that phrase lightly. To rate octane, testers use special single cylinder motors developed in the 1930s. In the United States, octane is reported as an average of research octane (RON) and motor octane (MON). That’s what that yellow "octane by R + M / 2" pump sticker means. In Europe, it’s RON only, so the numbers run higher. RON tests the fuel under load, but both numbers help engine makers match their designs to available fuel.

The rule, then, is use whatever octane rating the manufacturer recommends. A force-fed SAAB asking for 93 octane fuel really needs it. Fuel with less won’t be able to put up with the pressure of the turbo and temperature inside the cylinder. It will start sounding like the opening of Silly Love Songs, and no one wants to hear that.

On the other side, a Honda Accord is set up to use 87 octane. The environment inside Accord cylinders is not going push that fuel beyond its limits, so it can’t take advantage of the extra resilience of the more expensive brew. It’s added cost with no added advantage. Who wants to waste an extra cent on their fuel?

All rules are meant to be broken, though, which is why dropping gas grade to save money– or picking it up to increase performance– isn’t necessarily a joke. There are times when you may want to stray from the owner’s manual and, as is usually the case, only you can help you know when.

With some cars, there is no choice.  Carbon build-up inside an engine might mean you must use higher octane fuel. In older cars, you are the knock sensor and have to respond accordingly.   

Newer cars have their own knock sensors, like little U-boat commanders listening for pings. Upon hearing the noise, they retard the timing, firing off sparks when they can still do some good. That means reduced power and efficiency. So, while you’re saving at the pump by avoiding higher octane fuel, you’ll end up paying at the throttle.

Exactly how much difference any of this makes varies for every car. You can (and this is not recommended the author or anyone remotely related to TTAC) take a new Passat, run regular gas and not hear any blacksmithing from under the hood. You will also fail to get that squeal you wanted as the light turned green. 

Perhaps more importantly at the moment, your mileage may be off. Whether the drop is enough to warrant the extra cost of higher octane fuel is… uncertain. The only way to really know for sure whether or not premium fuel is worth the money: test out a couple of tanks of gas.

If a test is allowed. Some manufactures "recommend" premium fuel, others "require" it. For the most part, a car manufacture weighs mileage, horsepower and fuel grade against its perception of the ideal buyer. They want to put out the best numbers they can. Conversely, no company wants to tell you to fill up with 93. It’s an added, and inconspicuous, cost of ownership, one that owners are forced to confront often.

Bottom line: if a manufacturer asks you to spend more ON their car, not FOR it, you may want to laugh it off.

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49 Comments on “In Defense of… Regular Gas...”

  • avatar

    Quite a few cars that recommend premium can be run on regular without any issue other than reduced performance. Still, the difference is 20 cents in SoCal, probably less than $2 a tank for most high-performance cars.

  • avatar

    Last thing I had that required premium to not disintegrate itself was a 1977 Honda CT70. And that was due to tropical heat, low speeds and cheap fuel.

  • avatar

    Skimping on Octane to save a few bucks is plain stupid. My car requires 93 octane, it has never had anything else in its tank. In my area 93 costs 20 cents more than 87 no matter where you go. I generally get low 20’s for fuel economy and drive about 20K miles per year. 1000 gallons of fuel per year times 20 cents= $200 extra per year to have my car running properly and at peak performance. I would suspect that most people who work it out would also find the annual price tag of premium to be rather small. If you cannot afford a few hundred dollars for gas over the course of a year, you probably cant afford the car you are driving. I suggest a bycicle, moped or public transportation.

    Another thing, back when gas was $2.00 for regular, paying $2.20 for premium was a 10% price hike over regular. Now at $4.00 per gallon for 87 and $4.20 for premium, it is only a 5% hike. Premium gasoline has never been cheaper compared to the cost of regular. Bottom line, suck it up and buy premium (assuming your car requires it) if it is a recommendation, do the test, it may be more economical to buy the premium.

  • avatar

    Gamper hit my argument square on the head. My turbo requires it, so I swallow the price differential… which by the day becomes less and less (relatively).

  • avatar

    Since BJs was actually out of premium last week, I had to put 87 octane in my 2006 Trailblazer SS (LS2 powered). It requires 93 octane from the factory, but mine was retuned to REALLY require 93 octane (in exchange for a bunch more horsepower, of course).

    I knew it wouldn’t knock (the wizards at Vector Motorsports assured me of that – and they know more about LS2’s than maybe even GM), but shortly I’ll be able to record the mileage difference. In 3 years of owning the truck, I’ve never strayed much from 12.9 MPG. If I did a LOT of highway driving, I might get up to 13.5 (with a 4 speed and a 4.10 rear end, the interstate is NOT my friend).

    It definitely feels a little down on power right now (although 90 degree heat and 95% humidity saps the power on these, too)…soon I’ll be able to tell how badly the MPG toll is, though.

  • avatar
    night driver

    The environment inside 2003+ Accord V6 models actually is set up to push regular fuel beyond its limits — the compression ratio of the 2003-2007 Accord V6 is 10:1, and the 2008 was raised to 10.5:1. Honda simply trusts its knock sensors enough to sufficiently dial back the timing to safely run regular (with reduced performance), and knows its target market enough to not require premium.

    I drive a 2003 Accord V6 and can vouch for increased performance using premium as well as thoroughly acceptable performance using regular.

  • avatar
    Steven Lang

    I guess the words ‘required’ and ‘recommended’ rule the roost here. Most cars don’t need premium fuels and unless it’s ‘required’, you are pretty much paying a price premium to the oil companies.

    There is actually greater variance between brands than there is between octanes. Really. Also a higher level of ethanol can have a far greater negative effect as well due to it’s lower energy content.

    The detergent issue is also unsupported as it applies to premium vs. regular. Both of them have plenty of it and the carbon issue these days is not relevant. Today’s engines run far cleaner than those even a generation ago.

    However there is a bit of an issue with detergents provided by discount gas providers vs. name brand gas companies. If you fill up with discount gas, you may want to also give it a boost of Techron or another additive in order to help minimize any possible long-term effects.

    Alternatively, you can simply fill up for a couple pennies more at the name brand station and not worry about it. Given that the discount brands now use 10% ethanol in virtually all instances, I’d opt for the later.

  • avatar

    Octane is not “added” to gasoline; it’s an index against which knock resistance is measured. Back in the early 1920s Charles Kettering developed a rating system based on comparing fuels to a blend of iso-octane and heptane. Since iso-octane had good resistance to preignition, it was arbitrarily assigned a rating of 100; since heptane had lousy knock resistance, it was arbitrarily assigned a rating of 0. An octane rating of 90 means that a fuel has knock resistance equivalent to a blend of 90% iso-octane and 10% heptane. It doesn’t mean that it contains that balance of those hydrocarbons, just that its resistance to detonation is equivalent.

    Some fuels have better knock resistance than iso-octane or worse than heptane, so it’s possible to have an octane rating higher than 100 or lower than 0. Pure ethanol, for example, has an RON of 129, 116.5 pump octane, while toluene has a pump octane rating of 114.

  • avatar

    My older motorbike has a 9.5-1 squeeze without any computer management, knock sensor, O2 sensor, or fuel injection controlled by computer. 87 pings under load, 93 does not. So for 80 cents more a tank I use better fuel and get better performance, especially on hills.

    Most economy cars would not know 93 from 87 octane gas at all.

  • avatar

    (Some things also have an octane rating of less than 0, although you wouldn’t use them in motor fuel.)

  • avatar

    Where I live, ‘regular’ gas is 87 octane, but I made a quick trip out west a few weeks ago and saw some 85 octane and even some 85.5. What’s with that?

    I have a ScanguageII, and one of the things it displays is knock retard. As I understand it, knock retard is a retard of timing; i.e. less advance. There’s a steep but short grade I go up every day or so and I watch for knock retard. When I’ve seen it, I haven’t been able to see any retarding of the ignition that corresponds. The Scanguage will display 4 readings as chosen by the operator, so I usually have knock retard, timing advance and HP among the 4. From what I’ve read, even knock of very brief duration can damage the piston, so I’d think I’d be seeing some retard kicking in.

  • avatar
    Andy D

    Engines that require premium fuel over time have jacked up compression due to carbon build up. Getting rid of it is fun. It involves clouds of steam/ smoke and best of all, an Italian tuneup.

  • avatar

    Steven Lang

    After having seen the line of branded tanker trucks lined up for refueling from the same lines while working at several Philly area refineries, I have to disagree about the difference between gasoline brands. I did actually ask one of the petro-engineers what gives.

    While standing in direct line of sight of the refilling trucks, he explained that the refineries all make a base stock and the difference between brands comes from the additives. The tanker trucks are filled first with undifferentiated stock and then proceed to a row of additive tanks to get topped off.

    He also told me that the no-name gas stations that every metro area seems to have actually buy partial truck loads from branded competitors and mix them. It’s inefficient to both drive a partial tanker back to the refinery and to retrace your path to the same station when the mix of 87/89/93 gas on-board won’t fill their tanks.

    I have my preferred brands too, but the truth is that unless you know you’re station is being serviced by the same manufacturer’s refinery (e.g. Sunoco in Philly), then your getting the same stock available from the closest refinery.

  • avatar

    And here I was hoping for an amusing defense of leaded gasoline. It was either “Regular” or “Unleaded” during my childhood…

  • avatar
    bill h.

    There is one erroneous impression presented in this editorial–the current/recent model Saab cars of which I’m aware will make best use of full 93 octane, but they do NOT require it. The milder tuned turbo models recommend 90 or equivalent, but do not require 93.

    In any case, Saab Trionic engine management systems will dial back on performance if 87 is used, but 87 IS the minimum required octane rating, and many on the Saab bulletin boards use 87 with no reported problems.

  • avatar

    Thanks bill h.. I have been advised by SAAB dealer service departments (2), and my indie mechanic that 87 octane is fine for my 2 SAABs. All have said that they are more efficient with premium, but “you’ll never notice the difference”. My 2001 9-5 has 200k on it with no adverse effects yet. The 9-3 2.0 has only 93k.

  • avatar

    What good is the middle, 89, octane? With the conventional wisdom that refineries are running at full capacity would there be any benefit to phasing it out? Does it only exist to fulfill our consumer expectations of small, medium, and large choices?

  • avatar

    mel23, I understand that the octane sold in high-altitude areas is lower. This has something to do with less oxygen in the air.
    I heard that 87 octane with 10% ethanol actually has 89 octane because the alcohol increases octane. Does anyone know if this is true? In NJ, all gas is required to have 10% ethanol because of air pollution. I have been using regular in my Hemis (89 recommended) with no noticable difference. BTW around here, mid-grade is about 30 cents higher than regular, premium about .40.

  • avatar

    wookie76: I don’t think refineries make 89 octane. I think it’s made at the station by mixing the 87 and the premium grade. Way back when, I remember seeing pumps with a dial on the side that would make custom mixes between regular and premium. Clever, but nobody needs that level of fine-tuning!

  • avatar
    Gardiner Westbound

    In my experience the cash saving from substituting regular grade gasoline for premium where premium is specified by the auto manufacturer is minimal when the reduced fuel mileage is factored in.

    Notwithstanding Ontario gasoline tankers draw their supply from the nearest depot regardless of brand, recent local Shell Oil advertisements claim special benefits accrue from using its Tier 1 gasoline, including the absence of ethanol in the top premium grade.

  • avatar

    Some stations sell 89 octane right out of the tank, other stations sell fuel at 89 octane from a balanced mixture of 87 and 90 octane (or 91 or 92 or 93 or whatever their particular premium octane is).

    89 octane RON + MON / 2 “was” the prior “regular leaded” octane number and more or less has been sold to ensure that cars built from 1956 to 1974 tuned for “regular” leaded gasoline have an equivalent unleaded octane to (mis)fuel their cars with.

    Just like we are currently (mis)fuelling our gasoline cars with E10 (once called gasohol).

  • avatar

    BTW 89 octane was the prior RON + MON / 2 “regular leaded” standard octane for cars built from 1956 through 1974, and so it has historically been offered in an unleaded form so these folks can (mis)fuel their cars with unleaded. Not that I like the bad health aspects of lead.

    Naturally, as of now, most of us are (mis)fuelling our gasoline vehicles on E10 (previously known as “gasohol”).

    Some owners manuals (Mazda, maybe others) apparently specified NO ethanol should be used as late as 1989; yet this fuel is now mandated in multiple states and in many cities (by federal government rule).

  • avatar

    There must be people filling with regular instead of premium on the premise that with all the knock sensors and computers the engine will gladly adjust itself to a grade of gas with a possible minor degradation in performance and economy.

    The other point is simple, better 5 bucks in my jeans than an oil company’s jeans.

    Just like gas prices are “all over the place” is it really 87 – 89 – 92 octane?

  • avatar


    Agree with your reasoning as I have a 2000 Jetta 1.8T which has only ever seen premium and still running very well.

    However, we recently acquired a 2008 Passat 2.0T. Before we even asked, the sales guy told us we could put regular fuel in it. When they filled it for us before we left, they put regular fuel in. The manual however, recommends premium, as one would expect. What would you suggest in this case ?

  • avatar

    wookie76: +1 what bolhuijo said, and I’ll add that I am a customer at the 89 octane pump; the engine in one of my cars (without EFI) will ping with 87 but not with 89, so why should I pay extra for 91+ octane gas which I don’t need?

    night driver: Depending on the engine design, aluminum-headed engines can safely run up to an extra point of CR versus iron-headed engines without having detonation problems.

    Overall a very good article IMO. I have a few minor quibbles with it, but other commenters have already pointed them out.

    Regarding engines getting “carboned-up”, this became a big problem when MMT and MTBE replaced lead as an octane modifier. Combustion deposits from these chemicals don’t flake off the cylinder head and pistons as deposits from leaded fuel did. I suppose this is one advantage of ethanol being used as an octane modifier.

  • avatar

    I put regular unleaded in my Prius.

    I wish I could buy gasoline that was “Guarantted Ethanol Free.” Oh how I wish my motto could be “Corn is for eating!”

  • avatar


    That was Sunoco back in the old days. If I remember correctly, they had Sunoco 200 to 260, with the 260 being the 104 octane juice. All the knob did was blend the 200 and 260 grades to get the desired inbetween octance rating out of the hose. Good marketing trick at the time.

  • avatar

    @mel23 and essen – The lower octane out here in Colorado is due to less atmospheric pressure, not less oxygen. Just like the author of the article said, higher compression means higher cylinder temps which means earlier detonation. At sea level you have about 14.7 PSI of air pressure, that means in a 10:1 compression engine (at full throttle and no restriction in the intake), the cylinder pressure at TDC is 147 PSI and at 147 PSI you can get away with 87 octane gas. Now out in Colorado at, say 1 mile high in Denver, our air pressure is only 12.2 PSI, so in a 10:1 compression engine we only have 122 PSI. Since the pressure is lower you can get away with lower octane gas.

  • avatar

    The Placebo Effect. When I inherited mom’s ’86 Mazda 626, and occasionally decided to ‘splurge’ and put in the high octane stuff (this of course when gas was ‘cheap’), I would have placed my hand on a bible and swore that the engine ran smoother and quieter (not that I was hearing any pinging) but just in overall operation. Shrug.

  • avatar

    Gardiner Westbound

    I would be interested in learning more about gas in the GTA. Do you have any links for further reading? Thanks!


  • avatar
    Steven Lang

    “After having seen the line of branded tanker trucks lined up for refueling from the same lines while working at several Philly area refineries, I have to disagree about the difference between gasoline brands. I did actually ask one of the petro-engineers what gives. ”

    They get different gas. Most of the name brand firms get Tier 1 gas while nearly all the discount gas firms get Tier 2.

  • avatar

    One of my best friends has an ’06 Acura TL that ‘requires’ premium fuel. When she bought the car in 8/06, gas was in the low $2 range per gallon and the added cost of premium wasn’t so painful to the budget.

    As prices went up, she decided to experiment with lower-octane fuel. She dropped from 93 (Super) to 89 (Plus) with no noticeable impact on performance, economy and no audible pinging from the engine. The drop down to 87-octane (Regular) caused some intermittent issues under part-throttle, low-speed acceleration. So she’s been using 89-octane for the past year or so. It saves about $.15 per gallon.

    I wonder if carmakers will work to make all/most of their vehicles compatible with regular unleaded with current fuel prices? A few months back, I was considering an ’08 Acura TSX until I discovered that it required premium fuel. After visiting a few Acura enthusiast sites, it seems the TSX was not as forgiving as the TL in it’s appetite for premium. =(

  • avatar

    “A force-fed SAAB asking for 93 octane fuel really needs it”

    If it’s got a proper knock sensor, it should detect and compensate(reduce timing) to handle the lower octane fuel.

    “On the other side, a Honda Accord is set up to use 87 octane”

    You should hear these cars in the summer time driving up my street(I live on a hill). Ping city. Pretty bad when I can hear it clearly ~50′ away.

    The gas stations are another matter. I guess the fuel truck has compartments for the various octanes? I guess the guy puts the proper fuel in the proper tank at the station?

    I wonder how many times they have gotten mixed up? Accidentally put 87 in the 91 tank.

    Has some shady station ever filled all their tanks(87/89/91) with only 87 and sold and advertised the other two grades as usual?

    Have you ever been to a station that sold five grades(I have)? 87, 88, 89, 90, 91. Insanity. I think the trucks only hold 10,000 gallons. How many separate tanks?

  • avatar

    I don’t know where you guys live, but here it seems like there’s a 15 cent difference between regular and midgrade, and then an additional 10 cent difference between mid and premium.

  • avatar

    Not to be excessively pedantic, but the editorial should be edited for conciseness, to wit:

    “Octane (a.k.a. 2,2,4 trimethylpentane)”

    Should read “iso-Octane (a.k.a. 2,2,4 trimethylpentane)”

    (This was already pointed out by a previous psoter.)

  • avatar
    Andy D

    RTZ, as I understand discussions on this subject on BITOG, tankers carry just 87 and 93 octane. Intermediate grades are blended at the pump.
    Additive packs are added to individual tanker loads of generic gas

  • avatar

    I know someone who used 100% premium in a G35 until last summer. Then they switched to regular every third fill-up.

    Last winter they ramped it to regular every other fill-up.

    Now they only do premium every third fill-up.

    No discerned effect whatsoever in acceleration. No knocking ever, including under heavier load. No change in mpg.

    There are lots of internet people who will warn of dire effects, but I would really like to hear their empirical, tested results. The case I’m citing is an actual tested result.

  • avatar

    In my childhood it was regular or ethyl. Then Richfield out west added a high test called Boron. Of course back then the difference in cost was just pennies. 29.9 for regular, 31.9 for ethyl, and 33.9 for Boron. What exactly is Boron anyway?

    Richfield became Arco which I think is now Exxon.

    My favorite gasoline back then was sold in the midwest and had a Dinosaur on the sign.

  • avatar

    E10 is required in my area — it’ll be mandatory state-wide within a few more months.

    It seems to me that the ethanol in E10 could be considered “the super additive” — surely, nothing will clean your fuel system as well as 10% ethanol in your gasoline, right?

    So, if the difference between “Tier 1” and any other brand of gasoline lies all in the additive packages, then with E10 in the equation, “all bets are off.” Anything with E10 ought to clean the begeesuz out of your fuel system. So, buy the cheapest stuff you can find, and “Don’t worry, be happy.”


  • avatar

    I think that in many instances manufacturers’ calls to use premium is a crock. Why does a BMW 328 with a compression ratio of 10.8:1 call for premium but a CTS with a compression of 11.4:1 NOT need premium? The CTS produces more hp per liter on regular gas and with a higher compression ratio than the BMW on premium. The CTS proves that a high compression engine can be tuned to produce high power using regular gas. When BMW, Lexus, Infiniti and other companies require premium in engines with less compression than the CTS engine, they’re either doing it for image reasons, or they could have tuned the car to use regular with no loss of performance but chose not to, or they need to revisit their engine control technology.

  • avatar
    Dave Ruddell

    Regarding the chemistry of this whole business, n-octane actually has a negative octane rating (the longer the hydrocarbon chain, the lower the octane rating).

  • avatar

    Very interesting editorial

  • avatar

    Gottleib said, “My favorite gasoline back then was sold in the midwest and had a Dinosaur on the sign.”

    You must be thinking of Sinclair, the company founded by famed oilman Harry Sinclair, who spent six months in prison on contempt of court charges related to the Teapot Dome scandal during the Harding administration. The green dinosaur’s name is, naturally, Dino.

  • avatar
    Kevin Kluttz


    On the other side, a Honda Accord is set up to use 87 octane. The environment inside Accord cylinders is not going push that fuel beyond its limits, so it can’t take advantage of the extra resilience of the more expensive brew. It’s added cost with no added advantage. Who wants to waste an extra cent on their fuel?

    Honda Accords (at least the one I drive, a 2001 EX) have a knock sensor. I had one replaced under warranty. And YES, it DOES make a noticeable difference in this case. Same goes for my 1988 Grand Prix with knock sensor.

  • avatar

    This subject has been constantly debated on the internet for a decade.

    But this article is the most useful and informative I’ve yet seen. Nicely done.

    I can’t see the logic in using premium for any pushrod engine…but I’m no expert. It’s funny that few seem to buy the mid-grade. It’s either 87 or 93.

  • avatar

    My 2008 Mustang Bullitt has a sensor for octane and adjusts timing appropriately. Using a higher octane will provide a bump in torque.

  • avatar

    I have a question, I have a honda motorcycle (Honda Shadow VT1100) Its a simple twin carb set up…. Can I alter the engines compression ratio’s to get better performance… I don’t want to get a bigger bike for a little more horsepower! A Turbo perhaps?

  • avatar

    @pman: There are many variables in engine design, aside from simply the compression ratio, that will determine how tolerant it will be to running on lower octane fuel.

    @darcyb62: There is no “octane sensor” as such. What you are referring to is the engine knock sensor. If the computer doesn’t detect any preignition (knock) it will advance the ignition timing, which will increase available torque.

    @westhighgoalie: If you want to increase CR, you may be able to get a thinner head gasket, or remove the cylinder head(s) and have an engine machine shop remove some material from the face, or disassemble the engine and replace the pistons. A good starting place would be to find an enthusiast website for these motorcycles and inquire there.

  • avatar

    Thanks for clarifying Mike66Chryslers.

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