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Old 08-28-2008, 07:37 AM   #1
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Can some gallon gauges at fuel pumps be fooling us?

I drove 205 miles and filled up at 4.83 gallons. Thats 42.4 MPG with my stock Civic DX w/ AT and AC on. And I know that I was not using FE driving habits the whole time I drove the 205 miles. I normaly average 29-33mpg. More than likely the fuel pump was out of calibration? I guess I shouldnt be complaining because I got more fuel in my tank than what I really paid for.

Has anybody else been experiencing weird MPG numbers? Questionable higher or lower MPG numbers? Maybe the fuel pumps are to blame and not our vehicles? Its hard tell when a fuel pump is accurate.
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Old 08-28-2008, 08:48 AM   #2
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If I had to guess, I would say that the pump is accurate. that is money out of their pocket if it isn't accurate.

I would say it is because you underfilled your tank when filling back up. if you use the method that most use around here (pump until the first stop). there are differences in pumps stopping at the same place. they meter the fuel at the appropriate rate but may stop earlier or later than other. you may have had a short fill when you filled up or an overfill on the previous fill.
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Old 08-28-2008, 09:11 AM   #3
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There are a few factors here. Some work for us and some against us.

First, Fuel pumps are federally certified and inspected on a regular basis as part of that certification. If you look carefully, there's always an analog readout in addition to the digital meter, and there's a dated inspection sticker from a federal inspector saying that the pump is accurate. You can be pretty sure that the pumps are within federal standards for accuracy unless there's an out of compliance sticker (which I've seen before) on the pump.

Second, The fuel delivery rate affects the accuracy of the pump. It's not a large or consistent difference, but in general, slow delivery will tend to give you more than the metered quantity and fast delivery will give you less. It's a very very small difference though... and I have know idea why that is, but consumer reports (I think) tested it a few years back and proved it.

Third, and most importantly, the temperature of the fuel affects its density and volume. 1 gallon of warm fuel contains fewer molecules and less energy than one gallon of cool fuel. I used to do environmental surveys and on an inspection of a gas station I had to look at the fuel logs to try to determine if the underground tanks were leaking. The logs were so variable that it didn't make any sense at first. The attendant told me that based on fuel depth (which is how they gauge their tanks), a morning and an afternoon reading could differ by as much as 500 gallons due to the change in volume of fuel as it warms and cools.

So while the meter reading is correct to within a small degree of error and you really are getting a gallon of fuel, depending on the temp of the fuel, you might not be able to go as far on that gallon under some circumstances.

Lesson learned... Try to fill up in the early morning when the fuel is as cool as possible. You get more for your money at that time. (Also, use the slowest preset on the pump. Every little bit counts after all)
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Old 08-28-2008, 09:12 AM   #4
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there was a report a while back about pump inconsistancies. also, i heard recently about overcharging. a listener called into a radio station and claimed he pumped exactly 10 gal @ $3.55/gal and was charged $35.90 rather than $35.50.

perhaps it would be wise to verify volume pumped and price charged from time to time.
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Old 08-28-2008, 09:20 AM   #5
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Quote:
Originally Posted by bowtieguy View Post
there was a report a while back about pump inconsistancies. also, i heard recently about overcharging. a listener called into a radio station and claimed he pumped exactly 10 gal @ $3.55/gal and was charged $35.90 rather than $35.50.

perhaps it would be wise to verify volume pumped and price charged from time to time.
I could see how he would have been charged 35.60- because of the 9/10 cent extra added on to each gallon. But 35.90 is overcharging.
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Old 08-28-2008, 10:04 AM   #6
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a morning and an afternoon reading could differ by as much as 500 gallons due to the change in volume of fuel as it warms and cools.
How big were those tanks? To get a 500 gallon difference in 10,000 gallons of gas would require a 100 degree change in temperature. Normal diurnal temp changes is 20-30 degrees. Diurnal temp changes are imperceptable just a few feet underground and the bottom of the tanks (where the gas is) is easily ten feet down. I'm having a tough time with the temp change story.
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Old 08-28-2008, 11:14 AM   #7
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Same here. You are going to be hard pressed to find underground fuel storage tanks larger than 12,000 gallons. I was the night manager for a 7-11 in college and it was my responsibility to do the gas reports. I also was a manager in a fuel station for the US Marine Corps for a while as well. In both locations we had underground sensors to measure the temperature of the fuel in our tanks. The temperature of fuel in an underground tank does not fluctuate more than 1 or 2 degrees durring the day because its underground. Once you get about 10 feet underground the temperature is usually pretty stable year round. My parent's house is on a private well, where the water is taken from 300 feet underground. It does not matter what time of year it is, day or night, the water is always 55 degrees.

The volume of gasoline changes depending on temperature (as does virtually all matter). That is why you will see on a bill of lading from a fuel delivery truck gross gallons, net gallons, and the temperature of the gasoline at the time the truck was filled. Gross gallons are what the fuel station buys and pays for, but net gallons are what is pumped and you pay for in your tank. Gross gallons are how many gallons that quantity of gasoline would be at standard temperature & pressure (If I remember correctly its 29 inches mercury, and 60 degrees F.)

-Jay
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Old 08-28-2008, 01:35 PM   #8
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I could see how he would have been charged 35.60- because of the 9/10 cent extra added on to each gallon. But 35.90 is overcharging.
i would presume since he specifically advised the radio audience(and pumped as well) to stop filling at 10 gal, the 9/10 was accounted for.

i couldn't remember the exact amount except that it was 4 cents over on the gallon. the $3.55 was a guesstimate on current pricing.

perhaps the price was $3.549 and he was charged $35.90?
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Old 08-28-2008, 04:52 PM   #9
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Originally Posted by lowbridescape View Post
How big were those tanks? To get a 500 gallon difference in 10,000 gallons of gas would require a 100 degree change in temperature. Normal diurnal temp changes is 20-30 degrees. Diurnal temp changes are imperceptable just a few feet underground and the bottom of the tanks (where the gas is) is easily ten feet down. I'm having a tough time with the temp change story.
Granted, it's been a few years and my memory may be off. Sorry if it doesn't seem like it adds up... As I recall, it was a 10K gallon tank. I don't remember how deep it was. My concern at the time was whether or not it was leaking. It could also have been a measuring error on the part of the station manager (No offense Jay, but most of those guys aren't exactly rocket scientists). In fact, since the tanks are tubular and not straight sided (which would affect readings at the top and bottom of the tank drastically if you didn't account for it properly), that might be the case. But the temp/density issue is definitely true. And like Jay pointed out the traveler can get the short end of the stick if the fuel is over S.T.P. Even if it's just an occasional issue caused by a load of fuel from a truck that's been sitting in the sun all day it could account for some occasional fliers in your mpg calcs.
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Old 08-28-2008, 05:05 PM   #10
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Quote:
Originally Posted by lowbridescape View Post
How big were those tanks? To get a 500 gallon difference in 10,000 gallons of gas would require a 100 degree change in temperature. Normal diurnal temp changes is 20-30 degrees. Diurnal temp changes are imperceptable just a few feet underground and the bottom of the tanks (where the gas is) is easily ten feet down. I'm having a tough time with the temp change story.
Quote:
Originally Posted by Jay2TheRescue View Post
I was the night manager for a 7-11 in college and it was my responsibility to do the gas reports. I also was a manager in a fuel station for the US Marine Corps for a while as well. In both locations we had underground sensors to measure the temperature of the fuel in our tanks. The temperature of fuel in an underground tank does not fluctuate more than 1 or 2 degrees durring the day because its underground.
Thank you both! I hate seeing that misinformation repeated, for exactly the reasons described above. It's unfortunate that it's based on science and makes sense, but is missing vital info that totally disqualifies it.
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