Do 87 and 93 octane cost the same per mile driven

Factual Questions
1

…in everyone’s car, or just mine?

I’m wondering if this is some big scammy thing, or if my car is weird.

I get 14.5-15.5 mpg on 87 octane, the cheapest my gas station has to offer.

I get 16.5-18.0 mpg on 92, the most expensive, when driving in roughly the same traffic.

I’ve been tracking my mileage on regular for nearly a year, and premium for 5 fill-ups now, so I’m fairly certain this is a real effect, not an artifact of city/highways driven, tire pressure or weather.

With gas prices what they are at the moment, that comes to $0.25 per mile, either way. The benefit to the 92 is that I don’t have to fill up every 6 days, I can do it every 7. (Yes, I know I need a more fuel efficient vehicle, I really do. Don’t have the funds, however, so I have to make do.)

I’ve noticed that as gas prices have gone up, so has the spread between the octanes. That is, years ago you could pretty much count on mid-grade being 10 cents more expensive than regular, and premium 10 cents above that. When the prices started moving closer to 3 dollars than 2, that gap expanded, and now over 3 dollars, I’m not shocked to see a gap of 40 cents or more. Is this because it’s a percentage, and the gas sellers know that they’ll get me to spend just as much on regular because it’s the same per mile?

And a bonus question: are there any other benefits/drawbacks to using 92? Is it too “rich” for my engine or filters or anything? I don’t want to make my life more complicated in the long run by being more convenient once a week. I have noticed less hesitation with the 92 than the 87. It used to give me trouble at highway speeds when I’d have to slow down and then try to resume speed; sometimes I’d have to manually shift it into a lower gear so it could catch up. That doesn’t seem to be happening any more.

(I drive a 2005 Kia Sorento, if that’s useful information. Don’t know nothin’ bout the engine, though.)

2

Are you sure you are filling up the tank to the same level each time? Auto nozzle shut off varies a bit. When I was tracking it, I filled it until I could see the gas.

3

I used to work for a Honda dealership in 2005, and they used to tell their customers to stick to 87 unless their specific car required 92/93 (like the S2000). My Honda Civic would get worse mileage, like by 1 mile per gallon, on the good stuff vs the cheap stuff. What mattered more in my Civic and in my Chevrolet truck was what brand the gas was. Valero always burned fast, but Chevron gas burned longer.

The higher octane was designed for cars that would knock or ping due to compression firing the cylinder before the spark plug fired.

Obviously YMMV (and it does)

4

Gas prices are going down here. One of our cars requires higher octane, but gets much better milage than the 13 year old pick-up. Maybe we should try the higher octane in the truck. It’s only $0.20 more per gal, and we usually have at least that in gas points.
Yesterday, we paid $3.21 for regular.

5

All else being equal, unless your car requires or recommends a higher octane fuel, there’s absolutely no benefit to using it. The higher grades simply preventing knocking and preignition on higher compression or turbocharged engines. If your car doesn’t knock or ping with 87 octane, there’s no benefit to running higher octanes. On cars that are “premium recommended” (instead of required) the computer can de-tune somewhat to allow the car to run on lower octane fuel, and you might see a reduction in fuel economy that may or may not mean that running premium actually costs less. YMMV, as they say. I’m pretty sure a Kia Sorrento should be a regular gas only car.

However, these days in many places all else isn’t equal because of ethanol. Many gas stations sell non-ethanol premium gas, which would explain both the higher price difference and the higher fuel mileage you’re seeing with it. You might look closely on the pump next time you gas up.

If I may go off on a slight rant, I think this practice ought to be illegal. It’s like back in the 60’s when gas stations would put better additive packages and such in the higher octane gas to get people to put it in cars that didn’t require it. The FTC made that illegal in the 70’s and IMHO only selling ethanol-free premium is much the same thing.

6

(MPG / Cost per Gallon (87 octane)) * Cost per Gallon (92 Octane) = break even milage

So 15 MPG/ $3.20 = 4.6875 Miles/Dollar
4.6875 M/D * ($3.20 + $0.25) per gallon = ~16.2 MPG

Input your actual prices and MPG, you will quickly be able to see if it is a win or loss for for you. There are other incidentals like milage to the gas station, time at the station etc, but it is outside the scope of your question.

As for how are they priced. Quite honestly they are priced at what the market will bear. Depending on what kind of crude the hypothetical barrel came from it will have different oils and ratios of those oils in it. That oil is then separated (distilled) and some of it is converted to other oils of a shorter length (cracking). What conversion is done is again dependent on what the market will bear at the time. (yes, I am greatly simplifying things).

7

Slight hijack: someone told me that using 92/93 octane once in awhile was the rough equivalent of using a bottle of gas treatment. I gather that’s not true?

8

That’s actually a good point. I don’t top off; I stop when the pump stops. Still, unless the different grades are likely to stop at different places consistently, it shouldn’t matter over the long term tracking, right? That might explain some of the range of numbers within each category (about 1 mpg for 87; about 1.5mpg for 92 - although I’m fairly sure the lower numbers correlate to hellaciously slow rush hour weeks), but not the average difference being so consistent.

To figure out which I should buy, I do:

(Price of 87)/15 and
(Price of 92)/17

and see which is cheaper, and by how much. So far, they haven’t been more different than a penny; usually they are less than $0.005 different. That’s close enough to equivalent that convenience wins out and I use premium.

I admit, I’m not actually sure what “knocking” is, so I don’t know if it does it. Is it a knocking sound coming from the hood? It doesn’t do that. But sometimes when it hesitates reaccelerating at highway speeds, it sort of catches, shudders a little and…well…farts. Not a loud backfire or anything, just a little urp, and just one. I can’t really tell where it comes from, exhaust or engine. Putting it down a gear makes it stop doing that and gets it to accelerate. (But again, this hasn’t happened since I started using premium.)

9

Just another data point here, but my Cadillac gets its best mileage on 91 octane, but the cost per mile is cheapest using 89 octane.

Nope. The octane rating only indicates how compressible the gasoline is before it spontaneously combusts. The octane rating says absolutely nothing about the quality of the gas, any additives that might be present, or even how much energy it contains.

10

Oy. I hadn’t even considered that! :smack:

Next week, a new round of experiments with midgrade…for SCIENCE (and parsimony).

11

Knocking is a term used to describe premature combustion of the fuel in a cylinder. It makes a sound like someone is knocking on the engine, hence the term. Most recent cars have the ability to adjust their timing, injection and other factors virtually eliminating the premature detonation and .: knocking. However, when they make the adjustment they become less efficient, and thats why a car like yours gets poorer gas milage on lower octane fuel (Octane number effectively represents combustion point with higher octane fuel allowing for higher compression and timing). On a vehicle designed to run on 87 octane fuel, it does not adjust to improve efficiency with higher octane ratings, so it is a waste to run them.

As to the other question, some distributors put different detergent blends in different octane fuels (btw 89 is usually just 87 and 92 combined) so there is a benefit to those fuels if you need the detergent.

12

I always prefer to call pre-ignition ‘pinging’ rather than knocking. To me an engine ‘knock’ is a serious, physical malfunction (like a rod knock). Pre-ignition is more higher pitched and ‘tinny’ sounding than knocking.

As others have said it all comes down to compression ratios. My 04 Pathfinder says PREMIUM FUEL on the gas cap but when gas prices shot up I decided to use regular and see if it made a difference. It didn’t ping (modern cars’ engines’ computers can compensate) but it did seem to get worse mileage. I looked up my engine’s compression ratio and found it was 10:1. For those who don’t know that’s high compression and absolutely requires premium. Cars that don’t require premium will have ratios of 8:1 or less. And to reiterate- regardless of what anyone might tell you putting premium in a car that doesn’t require it is like throwing money in the trash.

By the way pre-ignition is also sometimes called ‘dieseling’ because diesel engines always make that sound because they don’t have spark plugs and ignite the fuel thru compression alone. Very high compression, diesel ratios are on the order of 20:1…

13

The Straight Dope: What’s the difference between premium and regular gas?

Question: Do the different “grades” of gasoline have different amounts of ethanol or other additives added to them? If so, that, rather than the octane level per se, could cause a difference in mileage.

14

While this may be true somewhere, I have only seen ethanol free gas sold as ethanol free gas, without regard to a octane rating. Further ethanol in gas increases the octane rating, and some stations have complained that they can actually sell higher octane gas by blending it themselves with ethanol then the ‘normal’ regular grade.

15

It’s not something I’ve noticed, but I’ll look for it.

16

I know I’ve seen it in Iowa, where I had to do a double-take when I saw 89 gas was cheaper than 87 gas, rather than seeing the usual 10 cent (or so) spread between the three grades. Turns out the 87 was ethanol-free, but the other grades were not.

17

92 AKI (eg USA ) = 95 RON (eg Europe/Asia.)

The use of 92 AKI in O2 sensor cars, eg cars since 2000 approx, should provide result in better mileage. It really should be regulated that the gap in prices, representing the cost of extra processing done to the better fuel, should be controlled by government. Actually, this only sounds socialist, its not actually socialist though… no need to be scared of being dragged into a McCarthyist witchhunt. (people have forgotten the name but are still scared of the idea of being called socialist ! )

18

Actually, I should correct this. The 89 was 10% ethanol. I don’t remember what the highest grade was.

19

Dieseling isn’t pre-ignition - its when the motor keeps running with the key turned off. I suppose it does have something to do with pre-ignition since the gas ignites without a spark.But it doesn’t make the same noise as knocking, it runs very rough and uneven. Haven’t seen a car do this in decades.

20

You haven’t seen it in decades because cars don’t use carburetors anymore. Back when they did, a mechanically-powered fuel pump brought fuel to the carburetor any time the engine was spinning, and the carburetor automatically delivered fuel any time air was flowing through it. So if conditions were such that fuel in the combustion chamber could spontaneously ignite, then the engine could continue to run even after the spark plugs were no longer sparking. FWIW I see this sometimes on my walk-behind lawn mower with shitty old gas and hot weather: the safety brake is trying to bring the engine to a stop, but run-on makes it continue to spin for several seconds after the ignition gets killed. I’m surprised the safety brake hasn’t been fried yet. :smiley:

Cars have had fuel injection since the mid-1980’s, which requires an electrically-powered fuel pump and computer-controlled fuel injectors. Now the fuel stops being delivered as soon as the computer stops operating the fuel injectors (i.e. as soon as you turn the key off). Presto: no more run-on/dieseling.