Knox

After plugging the 128th fill-up for my '06 TSX into my Excel spreadsheet, I noticed that I either run 92 or 93 octane; depending on which local fueling station I stop at.

With that said, I took the average MPG of my 92 fill-ups and compared it to my 93 octane fill-ups just to see if there was a difference between 1 octane level. For those that want to read up on octane ratings, go here.

92 Octane Results
Total Fillups: 76
Average MPG: 25.55

93 Octane Results
Total Fillups: 52
Average MPG: 27.16

This means that I average 1.61 more miles per gallon when running 93 octane.

For those that are statistical whizzes, was my sampling large enough to accurately justify a difference?

EuRTSX

I've been running 93 Octane since day one. It gets better everytime I fill it up.

I'm a 5AT and drive local everyday, and currently hit 23 mph.

I'd say not too shabby.

Knox

Most of my driving is 75% 3-5 mile commutes with the rest being interstate trips of 100 miles or more.

hunterk1

If you've got the full list of data (column 1, 93-octane mileage, column 2, 92-octane mileage, or something like that) I can do an F-test and t-test and tell you if the means are statistically different.

I'm not a statistician by trade but I think the sample size calculation would determine if the power was sufficient. I think I could do that too, but it might take me a little while to figure out.

nbtx

There are too many other variables like gas supplier, time of year, ethanol or not, etc. for those numbers to be statistically relevant. With 10.5:1 compression, I highly doubt the mileage difference is due to one octane point.

muncher

To know if there is any statistical validity to your sampling, you would also have to provide the standard deviation. in excel just do a =stdev() around your samples. then you can run a t-test to see. but judging by the numbers and high sampling rate i would say you are definitely on to something =)

also you have to try to eliminate as many variables as possible... ie the brands of the gas... or perhaps you got more 93 in the winter and more 92 in the summer.

either way, this is as scientific of a test i've seen on these boards. you made me proud =)

nbtx

Who were the two suppliers?

kevin1234

Hey. You think you could post up a picture of your excel sheet? I'm taking stats and am really interested to see your work

hunterk1

Unless he controlled for supplier and the other items you mentioned, perhaps more, he wouldn't be able to draw firm conclusions; but I would think doing the statistics will still say whether or not the difference is significant, which would be an interesting result.

Knox

Food City (not sure who their main gas supplier is) and Weigel's (main supplier Marathon). For those that want my data, send me a PM with your e-mail.

muncher

well at least you can say that one location gives better mileage than the other... and that's really what matters, right?

kevin1234

For some reason I cannot PM you (maybe cuz my account is new?). But my email address is gebby1160@hotmail.com

Knox

Price is what usually matters because the stations are so close. Food City, as noted above, does a "Fantastic Friday" promo where premium (92 octane) is the same price as mid-grade which usually results in a $.10/gallon savings. So, I usually stop every Friday to top off.

I'll send the data to those who have provided their e-mail addresses.

mknope

04 6MT - I keep all my fuel data in a spreadsheet and I average about 1 - 1.5 less mpg when I fill up with 91 octane as opposed to 93 octane. That's regardless of whether its winter or summer. Unfortunately almost all gas stations that carried 93 in my area are switching to 91 so now I have to go either to Mobil or Sunoco to get 93.

Sock

That presents a new hypothesis:
Food City is selling mid-grade as premium on "Fantastic Friday".

nbtx

Or 87 octane. My guess is the OP's 92 octane station is cheating customers by selling
87 octane from all their pumps. Mileage difference between 93 and 87 might be as much as the claimed 1.5 mpg, but you'd never see that much difference between 92 and 93.

davidspalding

Your gas stations sound like BJ's and Costco for me ... great price, but questionable source and quality. After switching to Shell (see the other thread(s)), even without using a calculator, I saw increased mileage, confirming (for me) that the warehouse stores have lower quality fuel.

Doesn't sound like EITHER is a top tier gas source, so I'd be skeptical of even quality control from month to month at those locations. I seem to recall reading on Car Talk or somewhere that places like "the Quickie Mart" (what I call "Stop 'n Robs") get gas where they can, cheapest, to make any kind of profit margin. Variations in supplier quite likely invalidate any statistical trends of buying 92 at one place, and 93 at another.

I could be fulla of course.

mattlissy

Does both stations use 10% ethanol. I wonder if 100% gasoline vs. 90% gasoline would make a difference in MPG? Both 93 Octane rating of course.

hunterk1

You bet it makes a difference, ethanol contains (citation) 23.5 MJ/L while gasoline weighs in 34.8 MJ/L. So it contains 67.5% of the energy that gas does on a volumetric basis. Energy density is the same reason why diesels get better millage, that fuel is more energy dense.

nbtx

Energy density is PART of the reason diesels get better fuel mileage, but the major
reason is a diesel does not run against manifold vacuum like a gas engine; hence less pumping loss.

Knox

Sorry for the delay is getting the data out guys. Life comes at you fast sometimes.

I have posted the data and other info on my blog here .

Mokos23

all i know is that 89 octane really does suck and zaps the power out of the TSX. i'm staying with 93 all the time!

aascsm

Costco

Why is the Costco gas station so much cheaper than other gas stations?

I always thought the gas station idea at Costco was just an afterthought… you know, a way for those guys to make a few more bucks. When I placed a telephone call to Costco headquarters in Issaquah, Washington, I found that I was dead wrong. Costco takes its gas very seriously. So seriously in fact, that they actually have a VP of Gasoline on their executive roster. (I wonder if they have a VP of Pizza-by-the-Slice?) I tried to reach the VP of Gasoline, but evidently he must’ve been out brokering a huge petroleum deal with Rotten Robbie’s, because my call ended up getting routed to a general information monkey. I must not be the first person who’s called with this question because the customer service rep read me a prepared script. It went something like this: “Costco is able to keep its gas prices so low because we don’t acquire our gas from just one source. We purchase gasoline from many sources (BP, 76, Arco to name a few) – wherever we can get it cheapest at that time. This way, we can pass the savings on to you.” So, if you don’t mind waiting in lines that hearken back to the gas crisis of ’73, you too can save a few clams at the Costco gas station.

http://www.thewavemag.com/pagegen.ph...ticleid=22544\


CBS News Report you might find interesting

http://www.cbsnews.com/stories/2005/...in788939.shtml

aascsm

BJ's

I think they also take it pretty seriously since they have a VP of fuel and automotive operations..

http://findarticles.com/p/articles/m...6/ai_n13781476

Mokos23

oh goodness i would never name my store BJ's.

Knox

I had a lot of people PM me for the Excel spreadsheet. Since I circulated it, I haven't seen any replies. Did my statisticians give up?

hunterk1

Ok, I'm back. Here are some results -

Grouping into 92 & 93 only (i.e. no station effect), there is a statistically significant difference. The F-test says the variances are equal and the 2-tailed t-test for equal variances is significant at p=0.0091. So the 93 fuel really is giving you about 1.5mpg better mileage; the means and standard deviations are

92RON - 25.5 ± 3.3 mpg (n=79)
93RON - 27.1 ± 3.6 mpg (n=53)

I didn't compute the (statistical) retrospective power, but I probably could. Give me a couple of days, I've been doing statistics for some grant applications all day and I need a break.

However it gets more interesting from there. I then compared the two largest station groups within each octane group (Food City, 92RON; Weigel's, 93RON), I found that the improvement in mileage must come from somewhere else, because, strangely enough,

92RON, Food City - 25.1 ± 3.1 mpg (n=74)
93RON, Weigel's - 24.9 ± 3.4 mpg (n=16)

plus they are statistically the same. So the improvement seen in the 93RON fuel appears to derive solely from the other sources. If I then compare the other sources alone (i.e. exclude Weigel's) I get:

93RON (no Weigel's) - 28.0 ± 2.1 mpg (n=37)

This is definitely statistically different from the whole 92RON group (p=0.002).

Bottom line: Don't buy Weigel's anymore!

fcatwo

I worked in the accounting dept of a major oil company a few years back and it wasn't unusual for us to trade gasoline with another oil company if it was beneficial to both to balance supplies in different area. It cost money to pump fuel 1,000mi through a pipeline. Also, oil refining is not something done by local small business so if there is only one refinery in your area they are likely supplying fuel to all the nearby stations. The trace additives the different brands brag about may be slightly different but it's highly likely they are added as the fuel goes in the tanker. JMPO of course.

There are two refineries side-by-side near where we live and we often ride by them when out on our bikes. I've never seen a tanker going in or out of either one with an oil company name on the side. My suspicion is that the trucks are privately owned and haul fuel much like trucks haul anything else. I also lucked into a tour of one of them and the engineer showing us around mentioned that they supply Costco. Obviously one or the other has to supply Costco. It's possible of course that some stations may be mixing 87 with their premium fuel to make a few extra bucks but I think most states have people who go around checking for such things. I doubt any refiner would buy into such a scheme so the station owner would have to work it out with the trucker -- certainly possible I suppose.

fcatwo

Just another wrinkle on the statistical stuff. Gasoline expands when warmer and the volume is adjusted for temperature when sold in bulk. You'll get a little more fuel if you buy from a cooler tank (say underground vs above during the summer).

fcatwo

The accuracy of pump readings can be a problem as well.

Knox

Wow! Awesome data buddy. Thanks.

From my own calculations I realized that the difference in price for Weigel's wasn't enough for me to leave Food City gasoline.

davidspalding

Wow, Hunterk1. All I can say is Wow.

I've heard anecdotally that buying gas in the morning versus evening can give you a little bit more, due ... to the pump's measurement mechanism? (I can't imagine gas "warms up" underground during the day, but ... I could be wrong.) Back when it was $1.75, who cared, but now ... I absolutely fill up in the morning. I did that once with my Jeep, drove 2-3 miles to work, and ... by midday the 18 yo 20-gallon tank had cracked. Had to replace it. Now I fill up and then drive 22 miles to work, so no worries.

After a tankful or two that I got 25.6 MPG out of (more city driving? more aggressive? a little of both I think), I'm back up to 28-30 MPG.

DuckDodgers

Actually, I have a couple of thoughts:

1) The 93 octane pump(s) other than Weiigel's) is/are possibly calibrated to deliver at or slightly over measured amount; the 92 octane unit (and Weigel's 93) is on the low end and delivering the legal minimum. Hypothosis check: When was each unit inspected last and what is the fine for underdelivery (the theory being that with all pumps aimed legally low, sooner or later one will exceed the legal allowance on the low side and get caught).

2) Is it possible that when you fill up with 93 9except Weigel's) vs. 92 (and Weigel's) you end up driving in a more fuel efficient manner (highway cruise, fewer lights, less traffic, etc.). Hypothosis check: What difference in your daily drives make you choose one over the other (probably answered by your discount Friday comment) becuae of your locations/duties?

nauticapinoy

I always do 93 for my baby, but a week ago I filled her to the top and let the gas go down to 0 on the range and I got the best gas mileage ever. I changed my lead footedness and only accelerate quickly a minimal amount of times imho. Also tried my best not to drive faster than 70 on the freeway. 70% of course of these miles are highway miles but I got about 31 mpg for a whole tank counting 480 miles for the tank not too shabby for me...

hunterk1

Thanks guys, I'm an engi-nerd and proud of it. I posted this at what, 9:20pm on a Friday night and thought... WOW - King Nerd!!I've heard that too, but it's because gasoline's density is fairly temperature dependent, so it's denser after cooling off overnight. The pump only measures volumetric flow, so in the morning you're getting more mass per volume (definition of density of course) and of course it's mass that's important when considering the total energy.Please note I can't say why there's a statistical difference. So if gas companies do trade product back and forth we should be surprised there is a difference, which leads us to...Both fine hypotheses. And you get extra points for actually noting that they are hypotheses!

Knox

The gas stations are literally side-by-side so there should not be differences in driving behavior based on where I fill-up. But, you did answer the ultimate question, Food City is my station of choice due to "Fantastic Friday's" when 92 is the same price as 89 ($.10 cheaper).

chuson

I've read this somewhere as well, but if you know, so does the oil companies. In our area (Toronto), gas price goes up about $0.05 after 11p.m till the next noon. Therefore, you still pay the same for that "extra dense" gas.

BTW: You guys are amazing, this is nice data collection and calculation to shut those people who said Octane 87 is literally no differences than Octane 91.

davidspalding

"Ignorance is bliss."

1Louder

Here's the other way to look at your data:



I tracked my mileage like this for 4 years with my former car, and I'm also with my 06 but don't have near the data points you do. I saw that weather affects mileage quite a bit. The most efficient (as I've experienced it) is warm weather, but not so warm you have to run the A/C all the time (however I've noticed that with the TSX the A/C does not seem to have nearly the impact as it did in my Civic). Winter usually tanks MPG. So this graph here is your average MPG on a given month, regardless of the year. This should track reasonably well to the average temperature you have in TN on a given month.

What's interesting about your data is that 93 octane is really only better than 92 Jul - Jan. Feb - June seems to be a push. Interesting also that 92 did not react to the climate changes like 93 did. I was just looking at pure octane ratings, not where you got it.

What would be interesting to add to your analysis is what kind of gas is being used at the stations you frequent - to see if they change seasonally (some do, depending on the climate).

You have great data here - enjoyed looking at it. However I think if you want to really draw some conclusions, you'll have to isolate more variables. For example, you'll want to use the same station, and even the same pump. The probem I had with long-term MPG analysis is the fact there ARE so many variables to isolate.

However you've got me thinking I should start looking for the 93 octane stations...

1Louder

Just caught this - this marketing strategy makes no sense to me. The optimum setup for any car is running the right octane for your engine - higher is not better. 92 in an engine that only needs 89 is questionable at best.