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Ok, so there's a gas station around here that is going out of business, so they are selling out all of their gas. They're out of the unleaded, so they're selling the premium for the cost of unleaded. My friends swears on her life that premium gas will give you better gas mileage, but I don't really believe it. I still go there to buy the premium stuff, just because I can do it for the same price as unleaded (plus 5 cents off for paying cash) but i don't really think it's doing anything spectacular to my car.

Does anyone know?
 

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If the engine is designed to run on regular you will see absolutely no increase in mileage using premium. If the engine is designed to run on premium you can usually get away with running on regular with no noticable drop in mileage. However, you will loose a little bit of power. Your friend is wrong. :)
 

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tanyaisntarobot, what follows is taken from a post I placed in another forum regarding this subject. Take it at face value....

Just an additional note regarding super unleaded as many people freak out about that. I've made full tank comparisons between regular and super unleaded in every vehicle I've driven over the last thirty years and I've always seen the same result: regular unleaded yields poorer mileage but is cheaper, while super yields higher mileage but is more expensive. In the end, cost per miles driven is virtually identical. Why do I use super then? Two reasons. First, I hate filling up so when using super my improved mileage can keep me on the road (depending on the vehicle) an extra day or two before filling up. Second, I have a relative and a best friend who work as chemical engineers at major refineries who state that although detergents are added to all grades of gas, super grades offer the most benefits.

TrailDust
 

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If the engine is designed to run on regular you will see absolutely no increase in mileage using premium. If the engine is designed to run on premium you can usually get away with running on regular with no noticable drop in mileage. However, you will loose a little bit of power. Your friend is wrong. :)

:bullshit2

Modern engines have knock sensors which will retard timing to prevent pre-detonation. This timing retard reduces power and efficiency. Higher octane gas will allow more timing before pre-detonation (knock) occurs, thereby increasing power and efficiency. If you custom-tune your engine and modify the fuel mapping and spark tables to increase your potential even more, but then you'll be required to run a higher octane fuel.

With all that said, I doubt you will re-coup the added cost of the premium fuel. I run 91/92 octane fuel and I might see an extra 1-2 MPGs, certainly not enough to cover the extra 20-22 cents/gal I pay for 92 octane. I run it for power, not economy reasons.
 

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Procrastinating on TN
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tanyaisntarobot, what follows is taken from a post I placed in another forum regarding this subject. Take it at face value....

Just an additional note regarding super unleaded as many people freak out about that. I've made full tank comparisons between regular and super unleaded in every vehicle I've driven over the last thirty years and I've always seen the same result: regular unleaded yields poorer mileage but is cheaper, while super yields higher mileage but is more expensive. In the end, cost per miles driven is virtually identical. Why do I use super then? Two reasons. First, I hate filling up so when using super my improved mileage can keep me on the road (depending on the vehicle) an extra day or two before filling up. Second, I have a relative and a best friend who work as chemical engineers at major refineries who state that although detergents are added to all grades of gas, super grades offer the most benefits.

TrailDust
+1 on this.

I will say that I've put premium in my little 4 banger before, and based on several 0-60 tests using both regular (87) and premium (91-92) I have concluded that using premium nets me exactly 0.00 RWHP. ;) :D My gas milage may increase by a tiny bit, we'll see for sure after I burn off the rest of this tank. Almost everygas station where I live, the price difference between lowest grade and highest grade is 20 or 21 cents. I usually buy about 15 gallons at a time, so that winds up costing me $3 more per tank, which for me averages out to about a whopping 8 bucks per month. Will I get good enough improvements in gas milage to more than cover it? Probably not. Is my truck suddently more powerful? No. IMO, on engines that are designed to run on 87 it really just doesn't matter, do whatever floats your boat. If your car is supposed to have premium, that's a different story.
 

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:bullshit2

Modern engines have knock sensors which will retard timing to prevent pre-detonation. This timing retard reduces power and efficiency. Higher octane gas will allow more timing before pre-detonation (knock) occurs, thereby increasing power and efficiency. If you custom-tune your engine and modify the fuel mapping and spark tables to increase your potential even more, but then you'll be required to run a higher octane fuel.

With all that said, I doubt you will re-coup the added cost of the premium fuel. I run 91/92 octane fuel and I might see an extra 1-2 MPGs, certainly not enough to cover the extra 20-22 cents/gal I pay for 92 octane. I run it for power, not economy reasons.

I'm quite aware of the ignition timing issues. Have you ever done any testing to see if that extra timing actually hurts economy? I know people who have, and it may drop economy, but by an immeasurable amount.
 

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It makes no difference GAS MILEAGE wise.....power wise a SMALL difference on engines that NEED higher octane. Its just throwing money away to use it really on any engine. The slight difference in power is not worth expensive gas nowadays. Its not possible that the gas mileage would be affected. The knock sensors will adjust to the lower octane and just reduce PEAK power somewhat noticably. It CANT affect gas mileage by more than the TINIEST amount.
 

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It makes no difference GAS MILEAGE wise.....power wise a SMALL difference on engines that NEED higher octane. Its just throwing money away to use it really on any engine. The slight difference in power is not worth expensive gas nowadays. Its not possible that the gas mileage would be affected. The knock sensors will adjust to the lower octane and just reduce PEAK power somewhat noticably. It CANT affect gas mileage by more than the TINIEST amount.
Then my suggestion to you is to conduct your own experiment and see. As I mentioned earlier, I've done this test on every vehicle I've owned over the past THIRTY years and have always seen the same results: Better mileage with super, lower with regular. And while we're talking about octane, knock, etc., here's and excerpt of something I posted in a different forum that directly relates to fuel and mileage....

Many people report poor or inconsistent mileage with the same vehicle. The big culprit in poor mileage may well be the gasoline blend. Most people think gasoline from Brand X actually came from Brand X's refineries, well, it very often does not. Gasoline is bought and sold on the open market and often not from your expected source. Also, when gasoline is produced at the refinery samples are constantly pulled for testing, much as master beer brewers pull samples, and the gasoline is tweaked in order to compensate for vapor lock, etc. In the end, the gasoline that leaves the refinery is only as good as the chemist who blended it. Finally, people also think that gasoline is produced and shipped immediately, but the reality is very different. Selling winter blend during the summer season (and vice versa) is common, and it's not just Mom & Pop small sellers who buy it cheaply off-season and sell it at those tempting discount prices, the independently-owned national brands do it too. This gas is produced according to demand estimates and stored in massive tank farms, then sold later when needed.

In summary, purchasing gas is more akin to playing the roulette wheel on a rolling cruise ship than anything else. If you can find a station in your area that consistently offers good gas, you're lucky. According to my sources there's two major brands that refine gas very well, and make efforts to ensure the stations that display their name also sell qualitative gas. Why have I not mentioned them?...I don't need a flame war over brands or a law suit from a brand either.

So take a look at where you're buying your gas, and at the very least don't buy it those nationally known discount warehouses or Mom & Pop stores that offer the tempting deals, as the saying "if it's too good to be true..." most often applies. Just food for thought.

TrailDust
 

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The only thing I think High octane does is help with knock and stop detonation. I don't really think it does more or less than just that. I know that high compression Honda engines have to have high octane or they start sounding like a diesel which sounds funny but will kill the car quickly. As for better gas mileage keep the fuel clean, and change filters. I also use the Lucus upper cylinder lube ever 3 or 4 months. Does anyone use this Lucus product? I personally like this and some marvel mystery I think they are good products thats just my opinion.
 

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Depends on the vehicle. I have tracked mileage for my 2002 Isuzu for 6 years. At 87 octane I get 13.4 MPG, average, at mid-grade 88.5 I get 16 MPG, average. (most stations have 87, 88 and 89 octane gas, but I found a few that sell 88.5 octane at Mid-grade prices).

At current cost of $4.37 a gallon for regular one mile costs me 33 cents to drive, at $4.47 for mid-grade one mile costs 28 cents to drive, so I save a nickel every mile.
 

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One time i acciedently filled up on 91 on a 5s-fe. I was pissed but i noticed the fuel guage dropped a lot slower. It all depends on the engine. The 5s-fe isnt designed for high octane gas but using it increases gas milage.
 

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On my 1MZ-FE I saw a drop in gas mileage when I filled up with premium. On one tank it could be just random chance.
 

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On my 1MZ-FE I saw a drop in gas mileage when I filled up with premium. On one tank it could be just random chance.
Samething happened to me. I'm gonna reset the ecu on my 1mz-fe and then keep running it on 91. So far i have refilled enough times that all my gas is 91.
 

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High octane = high resistance to premature detonation = higher power in a higher compression environment. In a low-compression environment, this translates to incomplete combustion and more crud left behind.

A tank of 93 in the 96 Corolla has always made it run a bit smoother, but mileage is either the same or slightly lower (over a tank of either entirely highway cruising in one day nonstop or a predictable week's worth of commute) and every time I've run higher-octane gas through there's been more gunk to come out when it's Seafoamed.

It's not gunk from the higher amounts of detergents, either, I've run Seafoam through the gas on 87 and then done the brake-booster for the cylinder head and not seen as much crud as 93 + Seafoam.

If you start hearing knock on an engine designed for regular, and you go to premium, that's a bandaid for the symptoms and doesn't address the problem. If you hear knock on an engine designed for premium and you're using regular, you're a dumbass and (arguably) so's your ECU.

The only time you should see a real benefit from an engine designed to burn regular running on premium is if you've made modifications such that your timing curve is drastically advanced, you've upped your static or dynamic compression ratio, or you've significantly increased combustion chamber pressures without correspondingly pulling timing (forced induction or nitrous).
 

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The only thing I think High octane does is help with knock and stop detonation. I don't really think it does more or less than just that. I know that high compression Honda engines have to have high octane or they start sounding like a diesel which sounds funny but will kill the car quickly. As for better gas mileage keep the fuel clean, and change filters. I also use the Lucus upper cylinder lube ever 3 or 4 months. Does anyone use this Lucus product? I personally like this and some marvel mystery I think they are good products thats just my opinion.
High compression engines ignite gas easier than lower compression engines, that's why high octane fuel is used. It burns slower than lower octane fuel. Sort of like a long burn versus a pop. Diesel engines have the highest compression and their fuel is heavier and use a different octane rating. Diesels don't use spark plugs and ignite the fuel by compression alone.
 

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Will this myth never end?

Premium gas actually has LESS potential energy due to additives that DELAY ignition. This is to prevent pinging in high compression engines. All other things equal, you should get slightly worse MPG with premium.

There is one exception. If the car's computer has a knock sensor that advances the timing to the knock point, then the car could get better MPG and power. I don't believe that "regular" engines have this feature. Only high perf cars have this.

In old cars, we would frequently get high octane and then literally rotate the distributer cap a bit to advance the spark a bit. It did give more power...perhaps 5%.

The moral of the story is this: If you car is not high perf and runs well on regular then get that. I would bet my left nut that the MPG would be the same in a controlled test. :D
 

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a little fyi the new models have knock sensors. and yes Premium can extend the mpg a little. Modern fuel injection and sensors make better use of the premium fuel.
 

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Will this myth never end?

Premium gas actually has LESS potential energy due to additives that DELAY ignition. This is to prevent pinging in high compression engines. All other things equal, you should get slightly worse MPG with premium.

There is one exception. If the car's computer has a knock sensor that advances the timing to the knock point, then the car could get better MPG and power. I don't believe that "regular" engines have this feature. Only high perf cars have this.

In old cars, we would frequently get high octane and then literally rotate the distributer cap a bit to advance the spark a bit. It did give more power...perhaps 5%.

The moral of the story is this: If you car is not high perf and runs well on regular then get that. I would bet my left nut that the MPG would be the same in a controlled test. :D
Hahahahaha! Well said.
 
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