I drive a little Hyundai around, and I’m out in the middle of nowhere, so all the roads are 55mph. With 93 octane, I get 32mpg, and with the 87 octane I get 28mpg. Basically, it works out to saving roughly 3/10ths of a cent per mile driven. So it’s just a tad cheaper, and I don’t have to fill up as often. Win/win.
Mileage is typically lower on winter gas. Most states require more oxygenated (i.e. ethanol or MTBE additives)fuels in the winter for smog/emissions purposes. Those fuels have lower energy content per gallon and thus yield lower mileage. Since your post states that you switched to premium in the summer - it might just be the summer blends increasing your mileage not the premium.
Obviously, if you’ve been religiously tracking the MPG for all those 158K miles - and it was always 18 no matter what the season - then perhaps premium is the reason for the increase.
On an unrelated note - American corn farmers and the US government are ripping you off much more than the oil companies are. Ethanol is subsidized $0.55 per gallon and protected by a 2.5% import tariff. In addition, Exxon makes between 12-14 cents per gallon of gas sold. The Federal government makes 18 cents/gallon and when state and local taxes are added approximately 50 cents of the cost of your gasoline goes to taxes. OBTW - the Federal gasoline tax was supposed to expire in 1978.
You are correct, it could be in a lower gear, but i assumed the driver would have mentioned that! If the car can’t maintain 70mph without downshifting (assuming level highway), then it does have engine problems. I guess a lot of carbon buildup or some such could cause detonation in the engine, resulting in a large power loss, which would be helped by higher octane gas.
what’s the problem? In your case premium is cheaper. Why would you hesitate?
I believe Tom and Ray are right about the higher octane for higher compression engines. I have personally found with my 1997 Buick Park Avenue Ultra (supercharged) that while the owner’s manual demands premium, I have been able to retain 90-95% of the power with mid grade. While this is not as much of a savings as regular, the big oil doesn’t get as much of my $$$ as they might want. To get my car to accept the mid grade gracefully and retain the power, even when stepping on it on the freeway, took multiple steps. The first thing I did was to clean the injectors as much as possible with the additive “Techron” not pro-guard with techron. After running three full tanks of premium with the techron, I filled up with mid grade, I happen to use mobil or bp or kwik-trip,I used a consumer purchasable diagnostic/resettable code reader. The good versions of these can reset the computer’s sensor logs to nothing so that the computer can relearn the new configuration. During the next three tankfuls I drove with a VERYlight foot on the gas. I gave myself time to get up to speed on the freeway(avoiding the really short ramps where possible). My car is supposed to get 18/24mpg according to the revised EPA guidelines. I am averaging 22-24mpg with a 50/50 mix of city an highway driving including leadfooting it some.
I hope this helps some other people with high compression cars maybe not give big oil as much of their money.
Your recommendation page on regular and premium gas is grossly irresponsible. To rely on a vehicle’s knock sensor to reduce timing is HORRIBLE advice.
How much experience do you have tuning? How many friends in the repair industry do you have? My friend had a turbo-charged GTI that he had to pay over $4000 out of pocket because he’s been using regular gas the whole life of the car (less than two years). A quick inspection of the fuel system showed he WAS NOT using premium gas. The result the was two destroyed pistons. The cost of the engine rebuild was totally on him, as he didn’t follow the manufacturers guidelines.
You write your article like this cannot happen. You only mention high-compression engines, not supercharged or turbocharged engines. Also, knock sensors are notoriously unreliable at high rpms due to the increased noise of the engine from all parts. This is also the the range of operation where disaster caused by detonation is likely to occur.
I wish he could say he read your article, so you could realize your folly in this. Fortunately for you, he was just being a cheapskate.
For someone who gets 20mpg and drives 10,000 miles a year, the cost of running premium is a scant $100-150 a year more. We pay MUCH more for insurance, and that doesn’t even cover a blown engine due to negligence. Is it worth the risk of what detonation can do to an engine??
Consider this. Say gas pricing is fixed, as follows:
85 octane: $3.00
87 octane: $3.10
89 octane: $3.20
It’s pretty easy to find this pricing scheme anywhere in town. Now consider the cost of gas for a year, based on the 10,000 mile national average and an average of 20mpg:
10,000 miles a year / 20mpg = 500 gallons
500 gallons x 3.00 = $1500 a year
or…
10,000 miles a year / 20mpg = 500 gallons
500 gallons x 3.20 = $1600 a year
If that argument doesn’t work, then consider this also. An engine requiring premium WILL pull timing to make running regular gas safer. This happens at a cost of horsepower and engine efficiency. This will have an impact on your mpg. Say it only affects it by a margin of -2 mpg, which is pretty conservative, depending on the engine. Some auto makers will pull more timing than others to make it safer. 3-4 mpg would be a safer estimate, but for the sake of argument, let’s say a 2 mpg loss.
10,000 miles a year / 18mpg = 555.5 gallons
555.5 gallons x 3.00 = $1666.50
Where did the savings go? It actually costed you more!
Do the responsible thing and update/remove your attempt at “impoverishing” oil company execs…
Oddly, I once owned a 1970 Toyota Hi-Lux (Pickup truck), which I bought used. When I got it, the seller explained to me that it really needed Hi test gas, which sounded odd, but it simply would not run well on regular. I think, if I remember, it had the 18RC engine (memory is fading fast these days).
I had it in the shop for some work one time, and the mechanic asked me why I “…had the octane selector cranked up so high?..” He pointed out a small clear plastic cap mounted on the sideof the distributor shaft, which when removed, exposed a dial that was cranked to a set of numbers, each indicating a certain octane number. It was all the way to high, and once I set it back down to a mid level, the little beast ran for 5 more years on the cheapest gas I could get…no complaints…no problems.
Maybe we need technology to go back a bit, huh?
I stumbled on this article by accident. Here in the UK, the most common petrol (gas) grade is 95 RON, referred to as ‘premium’. According to Wikipedia this is about equivalent to 90-91 US markings. All European cars are designed to run on this fuel. In the UK, fuel suppliers are required to meet British Standard BS EN 228.
We do have ‘super’ products too - these are 98 RON. Your Subaru dealer would recommend you run an Impreza WRX on super.
European cars tend to have higher power outputs from smaller engines. My 1.6 litre 2001 Ford Focus develops about 98 horsepower - the most recent model 1.6 litre with variable valve timing reaches 113. The 2.0 litre (top of the range, excepting the specialty turbocharged 2.5 inline 5 from the Volvo T5 cars used in the ST models) hits 143. It doesn’t give up torque, either, giving 136 lb ft at 4500rpm, comparable to the equivalent US engine.
You can’t lump every car and engine together.
If your friend trashed an engine by knocking 2 pistons out of it, then I would like to know if he had the stereo cranked up loud enough to drown out 2 years worth of rattling.
Did he choose to ignore the audible warnings and use regular anyway?
Did your friend have a faulty knock sensor or a more common fault which affects this problem - an EGR system fault?
Do a net search and read how octane ratings are figured. Compare the U.S. and Europe and see what you find.
In Colorado, Regular is specified as 85 octane, rather than 87. Local news says therefore to get Unleaded Plus (87) octane if your owners manual recommends 87 or Regular. Is 85 that much lower than 87 that it will cause any trouble whatsoever in most cars? We have an '05 Cavalier with a manual transmission and a '73 Charger with an automatic transmission, and they’re only driven in-town. Would using 85 rather than 87 produce any noticeable difference in either of those cars?
I think your reply was factually correct myself- - however the topic is whether or not premium is necessary in cars that specify premium fuel. Now, I could be wrong, but I don’t believe there are any cars out there specifying premium fuel when they’ll run just as well on regular. After all, the car company doesn’t get any money from the oil companies for gas - why would they care, from a price-of-gas point of view, what you dump in your tank? If they specify premium, it’s because the engine is designed to run on fuel with a higher burn resistance.
However, putting premium in a car that can run on regular can lead to carbon buildup and eventually the car WILL require premium.
I can’t say for sure, since I’m not at all familiar with that engine, but my guess is that the “octane selector” was actually a timing control knob. Turn it to low octane, and it’ll retard the timing.
Modern cars already do that - -they’ll reduce the timing for you instead of you having to turn a knob. But it comes as many have already said, at the expense of performance.
you’ll probably be fine on 87. Octane is lower at higher altitudes because there’s less oxygen, and therefore it won’t burn as easily in the first place. In other words, 85 there is going to have about the same burn characteristics as 87 at sea level.
The only time you’d need to fill up with 87 is if you were planning to go on a trip where you descended significantly below your current altitude.
Its pretty noticeable in the summer when my wife forgets and puts regular fuel into my Subaru WRX. You feel a difference and the engine pings a bit.
I ask my wife did you forget premium? She smiles and states yes when I feel the difference. This however is a highly sprung motor with HP coming on with a wallop at 3000RPM. I can see how a less peaky motor it would be less noticeable.
Octane
A petrol rating of how quickly the fuel burns. The higher the rating, the slower and more controlled the fuel burns. This allows its use in high compression engines.
I always thought this was the definition that higer octane flame travels like a .0001 of a second slower which converts to a few degrees of crankshaft revolution? Doesn’t the knock sensor just hear the ping then the PCM then retards the timing just enough till the ping goes away? So if the lower octane was defined as just more likely to preignite that would be because of the timing.
Maybe if, it takes the time of, 11 degrees of crankshaft revolution for the flame to travel across the combustion chamber then you could set the timing (at a certain rpm) at 10 BTDC (before top dead center)with no ping. Then if you use a higher octane fuel that takes 16 degrees of crankshaft revolution for the flame to travel across, then you could advance the timing to 15 BTDC with no resulting ping.
Lynn Auto Repair
Jackson TN
All this back and forth is shadowing the real truth: it entirely depends on your driving style and your car. If you like to slam on the gas pedal, are towing your 30 foot boat up the Rockies, or have a high compression ratio engine (> 10) then your car is more likely to knock. The anti-knock sensor on your car will prevent knock but is not a cure all. Optimal power comes when your peak pressure is about 15 - 20 CAD after top center. If you retard your timing so that peak pressure is occurring at 30 CAD aTDC to avoid knock then your engine efficiency becomes much worse and you start to waste gas. In addition, since you get worse efficiency you will compensate by increasing the throttle which increases cylinder pressure and in turn increases the likelihood of knock which can force an even worse retarding of spark timing from Maximum Brake Torque. Your car manufacturer has no reason to want you to buy expensive gas, so go with what they say, they make their recommendation on the gas they think will allow you to work at MBT. There is never any reason to use premium if your car manufacturer recommends regular unless for some reason your engine knocks or your live on a hill in Arizona with 0 humidity, have a car with anti-knock sensors, and have noticed poor engine performance. I should also note that anti-knock sensors actually try to run as close to knock as possible, they basically advance spark timing until you get knock and then retard a degree or two.
Also of note, knock is the result of auto-ignition of the unburned fuel air mixture ahead of the flame front. In regular flame combustion the flame spreads from the spark plug and is limited by the area of the flame front and the laminar flame velocity and turbulent perturbations in the surface of the flame sheet which result in a flame velocity on the order of ~5 - 10 m/s. Auto-ignition occurs when the pressure and temperature of the fuel-air mixture becomes high enough that combustion occurs without the impedus of a flame. The problem is that once the mixture gets to the condition where auto-ignition can occur, the combustion occurs much faster than in flame combustion which results in an expansion of the fuel air mixture at sonic speeds. This pressure wave (shock wave) bounces back and forth across your cylinder and the frequency of the sound is simply the speed of sound in the conditions in your cylinder divided by the cylinder diameter. There is no need for interaction between the flame front and the areas of auto-ignition. If diesel engines used pre-mixed charges then you would only have knock in your engine. Diesel’s only avoid knock by spraying in a liquid charge which forces a vaporization delay before the fuel can auto-ignite which prevents run-away auto-ignition.
I would recommend that if your car says to use premium and you do not live in Arizona or the Rockies that you measure your MPG under a normal drive cycle and then measure your MPG using 89 or 87 octane. The right gasoline is the cheapest gasoline that does not result in a loss in MPG. I know people are going to say that what really matters is performance, but I am going to tell you that if your car is milking every watt of power from your gasoline then you will have good driving performance.
The only time you’d need to fill up with 87 is if you were planning to go on a trip where you descended significantly below your current altitude.
That shouldn’t be a problem unless he has a very large fuel tank.
the engine was likely an 8RC.
i once had a new '71 toyota corona, and that was the engine it had. it DID require premium fuel, per the owner’s manual, as it had a high compression engine.
seemingly odd for a nominal ‘economy car,’ but high compression does allow for greater efficiency. and the car did get quite decent mileage. i chose this over all the competitors, because from the specs it had a larger (1.8L vs 1.5- 1.6L) engine and TALLER GEARING.
and thru the early '70s, gas was cheap anyway, even premium. i recall the octane adjuster on the distributor too. reset, yes it ran fine on regular.
(the 18RC which succeeded the 8RC came with a lower compression ratio, to more readily run on regular. maybe it could’ve been stuck in the trucks sooner than in the more ‘sporty’ car models.)
It said “can”, not will. There is wiggle room there. Of course it “can” but will it? Can in this instance is a weasel word.
In ye olde days, regular gas had very little detergent compared to premium gas. That’s probably where the notion of “gumming up” the engine came from. Nowadays there’s plenty of detergent even in regular. Don’t worry about it.