MPG difference with different gas?

I used to think that there could not possibly be any significant difference
in MPG from one gas brand to another. However I now do seem to be noticing
a difference.
When I use 76 Gasoline from my local gas station in Milpitas California I
seem to get:

20.5 MPG from my ’98 4x4 Nissan Frontier and 32.0 MPG from my ’88 Mazda
323 and 32.5 MPG from my ’89 Honda CRX

On the other hand when I use Shell gas from my local gas station, I get:
24.0 MPG from my ’98 4x4 Nissan Frontier and 37.0 MPG from my ’88 Mazda
323 and 38.0 MPG from my ’89 Honda CRX

That seems to be a 15-17% more with the Shell gas!!
I’m having a hard time believing it but I’ve already done the experiment
twice with each car and the results are consistent.
Can this be possible?

 

This usually should not make so much difference, but:

Is either 10% ethanol?
Is either a winter formulation?

Even though they're supposedly inspected, there COULD be a small
metering defficiency. With gas so valuable on a per gallon basis, it
wouldn't surprise me if there were some tinkering involved ...
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There can be a difference, but your results seem too dramatic.

MTBE additive reduces mpg by about 3% at the maximum concentration used
in metropolitan areas. (about 11% as I recall.)

Ethanol at 10% (the maximum allowed) reduces mpg by an *additional* 3%.

So, going from pure gasoline to ethanol would result in some 6%
reduction in mpg.

If your lower-mileage gas pump is off a few percent (call your state
weights and measures department), plus 6% loss from additives (you'd
probably have to cross county lines for this to happen), plus a few
percent change due to different branding or volatility..... I guess it
could happen.

 

it's possible, but 17% is a little extreme. 10% is much more within a
usual range. keep the experiment running and see how it averages out
over a longer period, then report back.

 

OK, I’ll keep logging the MPGs a few more times to see if the results
remain consistent. Actually, when I first noticed the difference on the
Nissan I thought that perhaps the engine was getting a bit out of tune, so
that is when I repeated the experiment with my other two cars and got
similar results.
My only other similar experience was a few years ago when we had an ARCO
station in the neighborhood. The ARCO gas station was using 10% Ethanol in
the winter months and I had then noticed a small decrease in MPG, around
5-6%, with the ARCO gas compared to Shell using my ’88 Mazda 323. I had
also noticed that the engine was running a bit rougher and had a little
less power with the ARCO(ethanol) gas, but in any case the differences
seemed small. I started preferring gas from other gas stations and since
then I have only occasionally checked gas mileage until now I noticed this
bigger difference.
I sometimes check MPG on long trips (ie. Primarily highway driving) as an
indication of engine health.

 

Thats strange, a car mechanic once told me that amongst the top tier
gas companies, 76 is supposed to be the best for Hondas?

 

Yes but...

You need to log a year with one brand, then a year with another, so as to
iron out any statistical oddities.

 

It's always possible with modern automobiles with computer operated
spark advance. If you buy gasoline with an octane lower than what the
engine requires, the computer will retard the spark thereby reducing
power and gasoline mileage. I can definitely see that when going from
87 octane to 85 octane in my Jeep. I usually stay away from 3-pump
stations (85, 88, 92 octane) for that reason. At those stations I
have to buy the middle grade which is much more expensive than the
regular at a 2-pump station (87, 92 octane.) Of course this varies by
location and altitude. I would trust the octane rating at a Shell
station a lot more than a neighborhood generic station. You may be
getting 85 when you think you are getting 87.

 

You may be

That's a great point. But it is also sometimes possible that you're
getting a higher octane than you may think. A tanker truck may be
emptied into a "regular" storage tank at the pump, rather than haul the
load all the way back to the distribution terminal.

 

Well, I think each tank truck must have a million-dollars in fuel on
board, so they are probably very careful where they put it.

I'm surprised they aren't being escorted with armored cars.

 

Nah, too "Mad Max".

 

Obviously, the more data, the better. But I don't think you need a
year worth of data to prove this point. Assuming the station visits
are alternated reasonably (eg. not Mobil in January and Shell in
July), I think you could pretty comfortable saying there was a
difference after three or four tanks of each if the difference were
15%. Think of it this way. After four tanks, that would be 150 miles
farther that the better gas would carry you. That's a pretty
significant difference.

His experience has convinced me that I want to try this experiment
myself. I don't expect to match his results (different cars,
different usage, different local fuel supplies, etc.) but it would be
interesting to know. I alternate between Shell and Mobil and I do see
inexplicable differences in fuel economy. I never bothered to try to
correlate it to fuel. Now that I think of it, I might be able to do
it retroactively.

Stay tuned.

 

A few tanks is inadequate because there are a *huge* number of variables in
fuel consumption.

As just one example, a side wind of a few miles per hour can result in a
dramatic reduction in aerodynamic efficiency. You could drive the same
route one morning, at the same pace, same traffic but with a side wind and
say return 37 mpg. The next morning, same temperature, same route, but
return 39 mpg... the difference? The side wind.

 

Since I keep a milage log of gas purchases in my car and I always pay
with the same credit card, I was able to look up where I had purchased
gas over the last year and check the gas milage obtained from
different stations. Unlike the previous poster, I did not find any
consistent or dramatic differences between the stations. I should
note that this was all premium fuel so it is possible that there might
be more variability with regular.

The car is a '94 Integra GS-R (5-speed) driven very hard (when
possible) in Chicago traffic. Usage of the car is typically very
consistent, about 250 miles per week. I couldn't find all the credit
card statements but I used all that I did find.

Shell Mobil Amoco Shell Amoco
Date #1 #1 #2 #2

06/12/2005 21.7
06/17/2005 24.6
06/23/2005 22.7
07/03/2005 23.2
07/14/2005 20.3
07/10/2005 21.5
07/19/2005 23.7
07/26/2005 22.7
08/08/2005 24.0
08/14/2005 21.6
08/22/2005 23.1
08/29/2005 22.5
09/11/2005 23.7
09/18/2005 20.6
09/26/2005 23.8
10/03/2005 20.8
10/13/2005 23.6
10/20/2005 25.5
10/26/2005 20.8
10/26/2005 20.8
11/04/2005 21.9
11/14/2005 22.4
11/20/2005 25.6
11/28/2005 23.7
12/05/2005 22.0
12/14/2005 21.6
12/21/2005 25.0
01/27/2006 24.5
02/05/2006 20.3
02/11/2006 23.3
02/19/2006 23.1
02/24/2006 26.1


Shell Mobil Amoco Shell Amoco
#1 #1 #2 #2

Visits 7 9 7 4 5
mean mpg 22.1 23.4 22.5 24.0 22.3


So I saw about a 10% difference between the best and worst station
which were both Shell stations. Also, not as much variation between
winter and summer as I expected. In fact, none at all that I can see
from a glance. Winter was pretty mild this year.

 

Well, I think each tank truck must have a million-dollars in fuel on

Lately, I've seen tankers going around in pairs, escorting each other.