do you really need to use premium gas?? (car rental, mileage, engines)

[M3 Mitch]

True enough, but 7 years ago the % difference between regular and premium (let's say regular is $1 and premium $1.20) is, well, 20%. You can forget about getting 20% better mileage out of a car by changing grade of fuel. So the premium provided only a small improvement in performance, and maybe some improvement in mileage but not enough to worry about.

Fast forward to today. I'm paying 3.70 for regular around here. Premium is, strangely enough, still about $0.20 per gallon more expensive - round the prices off to $4/gallon and the 20 cent difference is about 5% - you can definitely get 5% better mileage from premium *in a car designed for premium*.

FWIW, our Subaru Legacy gets noticably better mileage out of premium.

In any case, the 5% cost increase for premium is small beer compared to other things that can affect the cost of driving a car.

[Deez Nuttz]

Originally Posted by Stratford, Ct. Resident

Note that 7 years ago, knock sensors prevented detonation from occuring in the tested vehicles. The technology has advanced further since then.

Knock sensors do not prevent detonation. Knock sensors tell the ECM to pull back some timing when they detect detonation, in hopes of preventing your engine from grenading all over the pavement.

When the sensor goes off the damage is already being done.

[Coldjensens]

I had a friend who owned a gas station that told me that the grades of gasoline are all the same. There is no difference in the octane even though it says so on the pump it is just a scam. No idea if that it true or not.

My understanding is that using lower grade gas will get you "pinging" which will last until you fill up again with higher grade gas. I used regular in my camaro once in Mexico (no premium available) and had no problems, but I never was comfortable enough to risk using lower octane gas. I like that car too much.

[Sayantsi]

Yes, modern engines can retard timing to reduce detonation when you use lower octane fuel. They have to detect it first, so by putting lower grade fuel in your car, you are causing some damage through detonation.

Running the recommended octane delivers the best power and gas mileage. Saving a couple cents on gas when filling up doesn't equate to the loss of power and mpg. Running a higher octane than recommended usually does nothing for an engine since the ECU isn't mapped for it.

Some very new engines (certain GMs, 2008 and on) are being designed to run on regular but up power when they detect higher octanes - this allows more power without the detonation issues.

The post above on the premium cost as a percentage of total cost is excellent. Premium costs 20¢ more than regular, plus 10¢ more. It always has, and continues to. The base price of a gallon is going up, so you're paying more for the gas no matter what octane you buy - it doesn't make sense to use lower octane unless you're really that intent on saving money when filling up at the expense of power, engine wear, and lower mpg.

[f_m]

Originally Posted by Stratford, Ct. Resident

Regular or Premium? - Feature/Features/Classic Cars/High Performance/Hot Lists/Reviews/Car and Driver - Car And Driver (http://www.caranddriver.com/reviews/hot_lists/high_performance/features_classic_cars/regular_or_premium_feature/%28page%29/1 - broken link)

Some of these parts of the article make sense to me.

"We should note that even cars designed to run on regular fuel might require higher octane as they age. Carbon buildup inside the cylinder can create hot spots that can initiate knock. So can malfunctioning exhaust-gas-recirculation systems that raise cylinder temperatures. Hot temperatures and exceptionally low humidity can increase an engine's octane requirements as well. High altitude reduces the demand for octane."

So the condition of the engine system will affect the result. Such that using premium may make up for the lack of the engine being in 100% proper condition.

Also:
"A Honda Accord V-6 with VTEC ... The Accord took a tiny step backward in power (minus 2.6 percent) and performance (minus 1.5 percent) on premium fuel, a phenomenon for which none of the experts we consulted could offer an explanation except to posit that the results may fall within normal test-to-test variability."

Doesn't necessarily seem that surprising or improbable, in that the VTEC engine may be specifically tuned for regular grade.

It's also not surprising they found lower performance with regular on vehicles that are supposed to use premium.

I do think there are some variations in grade and brands, not necessarily on purpose, but by random conditions (mixing, etc...).

Here are a couple more articles:

Regular Versus Premium Gasoline

Fact or Fiction?: Premium Gasoline Delivers Premium Benefits to Your Car: Scientific American

[bhcompy]

Originally Posted by Coldjensens

I had a friend who owned a gas station that told me that the grades of gasoline are all the same. There is no difference in the octane even though it says so on the pump it is just a scam. No idea if that it true or not..

that may be true for some stations, but not typical

[Drover]

Originally Posted by wowbaby

if it says so in your manual? i was told when i bought my mini, (which was supposed to take super) that as long as you pick a grade and stick to it you were ok. it is the switching back and forth that causes the problems.

any truth to this?

also does it make a difference if it is a turbo charged engine? i dont want to ruin my (new) car cause i am too cheap to use premium gas, but if one is as good as the other - well, DUH no brainer!!

thanks.

I would never not use premium in a turbocharged car, especially if you drive it the way it's intended to be driven. Under moderate to heavy load, you're already shoving hot charged air into the combustion chamber before it ever even gets to the compression stroke to make it even hotter.

Here's what I've discovered putting regular gas in a car where premium is "recommended": the timing gets dialed back so much that fuel efficiency drops to the point where any savings gained by using a lower grade are more than wiped out by lower fuel mileage. This is especially true when gas is more expensive because the more expensive gas is, the smaller the price difference between each grade as a percentage. Ironic considering it's more expensive gas that usually pushes more people to use a lower grade.

[Drover]

Originally Posted by Stratford, Ct. Resident

Bottom line: You're not buying "insurance" when you purchase 93 octane in lieu of 91 octane. You're just adding to big oil's profit margin.

I don't recall ever seeing a pump where I had a choice between 91 or 93 octane. Usually the choice is between 87/89/91, or 87/89/93, depending an awful lot on where you live. In many parts of the country, 91 octane is rare. I live in one of those parts, and so it's always 93 octane octane for me (though every once in a while at a "cheapie" station the highest is 92 octane) because 98% of the time my next available option is 89, which isn't worth saving $4 a month for.

[Stratford, Ct. Resident]

Originally Posted by Drover

I don't recall ever seeing a pump where I had a choice between 91 or 93 octane. Usually the choice is between 87/89/91, or 87/89/93, depending an awful lot on where you live. In many parts of the country, 91 octane is rare. I live in one of those parts, and so it's always 93 octane octane for me (though every once in a while at a "cheapie" station the highest is 92 octane) because 98% of the time my next available option is 89, which isn't worth saving $4 a month for.

Re-read page 1 of this thread. Or don't. Exxon/Mobil will thank you for doing the latter.

[Drover]

Wow, what an incredibly insightful and thought-provoking response. Thank you for being such a useful part of the conversation.

I'm sure the CEO of ExxonMobil has bought himself an extra condo in Vail on account of the extra $4 a month I spend to make sure my cars don't run like crap.