Comparison of MPG from different gas stations

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Sep 16, 2004
Santa Barbara, CA
Data is taken since 6/2003. Visits to the same station were grouped together, couple outliers were excluded (one 38.8 MPG data point - the tank consisted of almost entirely downhill miles 8000' to sea level). I go to shell for the 5% rebate on gas since 10/03 whenever is possible.

Y error bars are +/- the standard deviation and as you can see, they all overlap. Therefore I cannot find any statistical difference between these gas stations that I use. It elimates one variable when comparing MPG tank by tank, at least.


average MPG | std dev | #samples
exxon | 33.14 | 0.90 |12
shell #1 | 33.52 | 1.54 | 15
shell #2 | 33.19 | 0.84 | 12
shell #3 | 34.07 | 1.06 | 55
shell #4 | 33.59 | 1.25 | 9
shell #5 | 34.18 | 0.92 | 12
vons gas | 34.12 | 0.81 | 7
Way back in the late 1960's I worked for Exxon. At that time, their gasoline had the lowest specific gravity of all the majors, meaning that a gallon contained less fuel by weight than all other gasolines. As a result, their fuel economy in sustained driving was the worst. Amoco was the best, and it was also the heaviest gasoline.

Exxon used to justify this by saying that, due to the gasoline's low weight and high vapor pressure, it resulted in faster engine warm ups, so that the engine got up to operating temperature sooner. They then went on to say that in "real world" short trip driving, getting up to optimal operating temperature sooner meant that the car got to optimal operating conditions sooner, and therefore there really was no mileage penalty.

Most of we engineers there thought this rationale was a crock of ****.

Today, with more stringest regulatory limits on what can go into gasoline and its properties, I would expect the differences between brands to be less, but perhaps not zero.
Surfstar, thanks for presenting your results to everybody. This is the kind of testing "Consumer Reports" should be doing (instead of stuff like spagehetti sauce). Since they don't, it is up to individuals. We're all lucky we have BITOG to share it.
When discussing MPG variance, a lot of people report "I get more with brand X than with Y". I'd like to see them really compare the brands as I've presented. So at least in my region (stations varied as far as 100 miles) of CA the gas stations are within range.
I've found pumps to be less than precise on gallons measurement.

I used to fill portable gas containers of a 5 gallon size all the time. What I found was that when the pump said 5 gallons, the actual amount registered on the container scale would be significantly less or even more than 5 gallons. A couple of times the variance was 1/2 gallon per 5 gallon sample.

You need to bear this variance in mind when trying to get a precise MPG calculation.
merlin64: Allow me to suggest that the bulk of your observed error may not have been with the calibrated pumps, but rather with the uncalibrated gas cans.
Gasoline has a high coefficient of thermal expansion, so the gas can levels would be different depending on fuel temperature. The ratio is something like 0.0006 gal/F. Not enough to account for your 1/2 gallon observation though.
gasoline/diesel from underground storage tanks is probably 68f +/-2f anywhere in continental USA. The change in volume due to temperature as it goes thru the pump's metering gauge are nil.

All of the expansion due to temperature would occur after it gets into the can (or the car's fuel tank), or at least at some point after pumping.
I have a trip computer and have been putting exclusively BP gas since it does not come (hopefully) from LA area. MPG has been up. I get 22-23 in my V6 AWD auto wagon.
"merlin64: Allow me to suggest that the bulk of your observed error may not have been with the calibrated pumps, but rather with the uncalibrated gas cans. "

Actually, they were calibrated to the 1/4 gallon, and were semi-transparent so the level could be easily refrenced.

Try it youself with the same container at different gas stations. You will be surprised.

Actually, this finding should not be a shock. Surely each pump has a certain engineered tolerance. Then add time and wear. You think you will get a perfect 10.45 gallons on every pump in America with no variances?
Well, I know how often and how precisely the pumps in this State are calibrated and what to expect from them. You're certainly correct that there will be some variation. Perhaps your State does not check gasoline pumps as much as mine(?)

Calibrated to 1/4 gallon? Are you saying they are calibrated and the mark on the side is certified to be within +/- 1 quart of what's inside? Or are you saying there are marks on the side for every quart?
The pumps here in Tulsa are checked annually. The inspection sticker will say something like 87 -1 89 +2 91 +0 - the second number reflects the number of tablespoons of gas dispensed above or below the amount shown on the pump display.
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