During the past 4 months or so, I have been running comparisons between cheap gas (mostly Arco) and expensive gas (mostly Shell). I have since driven many tanks in a row of Arco, then Shell then back to Arco with both my hybrids.
I cannot come to a conclusion if Arco gives me less mileage than Shell or not, but from my test the difference can not be more than 3%. I would consider 3% sufficient, since Shell is less than 3% more, but in terms of mileage per dollar I am leaning toward Arco, since the 3% are within the error margin of my tests.
I was wondering if any of you had come to a similar conclusion or if I am way off.
Thanks

 

I use the cheapest gas I can find. Across several cars since the early 70s to include a Suburban now at 427,000 miles on the odometer, I cannot tell the difference between brands. Keep in mind that jobbers sell bulk gas between them when necessary and convenient. The other stuff they advertise is a few gallons of "stuff" added at the retailer when they fill him up.

 

I tend to agree with Bill, although I have no statistics or science to back that up. My understanding is that motor fuels meet an ASTM spec and are augmented (or not) with proprietary blends of detergents and other additives specific to the retailer. These blends are on top of the oxygenate (such as MTBE or ethanol) that is blended with the fuel after the refining process is complete to meet the various EPA emissions requirements around the country.

BTW, what was the standard deviation on the average mpg for both the Arco and Shell mpg datasets?

There are lies, **** lies, and statistics...you could normalize for average temperature, AC usage, urban vs highway driving per tank, etc., etc...

 

Excuse me, I mean "lies, darned lies, and statistics..."

 

If I take my overall variation of best tank - worst tank (approx 43mpg - 37mpg = 6mpg) as 6sigma, I get a sigma of 1mpg (about 2.5%)
I am actually on my best tank yet, which I attribute to me becoming more familiar with the Camry Hybrid (my wife now rides the Escape) and the higher temps. I filled this one at some discount shopping gas station.
The Escape is usually between 27-30mpg, although I have been doing 24 with a roof top tent pushing headwind, but that is a different story.

 

I've thought that trying to use the analysis tools (standard deviation & such) was doomed to fruitlessness. Without the ability to accurately measure the amount of fuel put into the tank to that level of precision I just couldn't get there.

So my approach has been to record the mileage on the fuel receipt and allow the math of ever larger gas & mileage numbers to erase the need for precision. For example my Suburban, the one with 427,000 on the odo, has so much gas & mileage that the imprecision of fillup to the pump cutoff is insignificant.

Along the way I just couldn't discern much difference in fuel mileage between producers. I've even tried to track spark plug performance across ~50k miles. Saw only .1 to .2 mpg difference.

Bottom line: it is known that petroleum has ~18,000 btu/lbm (forgive the ancient numbers & units but that's what I had to learn in college in the dark ages - so they stick) and that doesn't vary much across any given gallon. So if any given gallon has virtually the same energy, fuel mileage in a properly tuned car should be virtually the same. (In these days of computer controlled spark & fuel control what car isn't in pretty good tune?)

Since the gas companies practice "zone pricing" - I practice zone buying!

 

FWIW new user here with about 10 yrs
experience in the gasoline distribution
industry ( 70's ) but it's all pretty much
the same today. The blending is done at
the tank farm. The delivery trucks tanks
are sectioned off and each section is filled
for a specific deliver point. That would be
an in ground tank at a specific station.
The same tank farm can deliver fuel to many
many different brands. It's the additives
that are added when the tanks in the delivery
trucks are filled. There is a _LOT_ of exchanging
fuel. One brand in say Virgina may exchange
fuel with a competitor in Alabama. The base
gas is the same, it's the additives that are
different. My 06 FEH recommends 87 Octane but
I use 89 as a rule. The engine will run somewhat
cooler and be less likely to ping. If you are
going on a long trip on a hot day use 91 Octane.
That will tend to run the engine slightly cooler
yet. You get NO additional horsepower by using
the higher Octane fuel.

 

[quote=wilcal;My 06 FEH recommends 87 Octane but
I use 89 as a rule. The engine will run somewhat
cooler and be less likely to ping. If you are
going on a long trip on a hot day use 91 Octane.
That will tend to run the engine slightly cooler
yet. You get NO additional horsepower by using
the higher Octane fuel.[/quote]

A higher octane fuel than 87 will decrease MPG in the FEH because higher octanes have lower BTU. If the engine is running at the normal operating temperature there is no need to run the engine cooler and less efficient. The FEH has an Anti-Engine knock program to adjust timing to eliminate pinging. I have had some pinging near max RPM because of E10 but never with straight 87 octane. Ethanol also reduces BTU in straight gas and also lowers MPG in the FEH.

I prefer to use in my FEH's the cleanest cheapest straight 87 octane fuel with no additives such as the higher priced brand use. The type of fuel injectors the FEH has are self-cleaning and there is no need for additives. In fact, Ford recommends no additives be used in the fuel including alcohol which reduces the quality of the fuel that will effect Long Term Fuel Trim Maps. I also prefer to use a high volume gas station like Sams Club to get fresh gas all the time.

GaryG

 

Out here where I drive the octane rating of "regular" is 85. I use that exclusively in my FEH & Suburban. I see little difference in performance.

Its not accurate that higher octane fuel has less energy: its the same. The octane rating comes from additives, used to be tetraethyl lead, don't know what they use these days in the no lead world.

If alcohol is added it has a higher octane but just above half the energy of gasoline. E-10 will result in ~95% of the energy per gallon as straight gas. On a scale of 15-17 mpg in my Suburban I see no discernable difference in performance and my FEH seems about the same.

You know, speaking of 6 sigma just seems a reach for extra precision that isn't really attainable. Sure you can run the numbers, but just how accurate is your odometer & the fuel pump at the service station? I am skeptical of too much precision, since I see the input numbers as not as accurate as your calculations.

 

I have purchased gasoline in 44 states over the past 5 years so I can speak to this with some authority.

BUY THE CHEAPEST and LOWEST OCTANE POSSIBLE.
Quality varries by region of the country, but not by brand.

Higher Octane can, and many times, maybe always, have lower btu.
Higher Octane can, and many times does, contain higher % of ethanol.
Ethanol has lower btu, but much higher octane = anti-knock properties.

Now, fancier engines that need higher octane have higher compression.
Higher compression takes more advantage of the ethanol in "premium" higher octane gas, so while the btu are slightly lower (like 3% lower) the MPG will be 3% higher, = no net difference.

I have used 85 octane almost exclusivly in my FEH. With the FEH's 550-600 mile range per tank under ideal conditions, I can fill up with 85 octane in Utah and drive to sea level and have never noticed any knock or ping... at sea level, with 85 octane gas...

I also tow with 85 octane, in summer, and my engine never gets more than 10 degrees warmer, and those 10 degrees actually make the engine more efficient.