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kman2424 10-18-2012 10:12 PM

Gas Pumps Being out of Calibration?
Today I filled the tank on my truck the EVIC was saying 18.0 mpg but after I filled up the Actual number was more like 16.8 mpg (353 miles / 21 gallons filled to the first click) Now that is about a 6.7% error in fuel economy reading. No Surprise there seeing how that seems to be a normal finding on this forum. Also when i was getting gas I had a 5 gallon gas can for my lawn mower can and i filled the the can exactly on the 5 gal mark and the the gas pump said 5.25 gal which is about 4.8% error in the pump. So if my calculations are correct the percent error in my EVIC is more like 1.9% or about 17.65ish mpg (corrected for the pump being off) which sounds correct.

I guess my question is has anyone else found this to be the case when they by gas and put it in a gas can?

If so maybe the EVIC isnt as wrong as everyone says it is or maybe is just don't know how to do math lol and I'm horribly wrong

I did do a little searching just didn't find what i was looking for.

davidgcet 10-18-2012 10:18 PM

5 gallon cans will hold more as they age or on warm days, they stretch.

the EVIC figures avg mpg based on the last 2-3 hours of driving, thus why it is usually off. mine will often show 10 mog for the avg because each morning i crank my truck, drive out to the barn to take care of the horse and leave it idling, drive 3 miles to work, then drive home, idle at the barn for 10 mins, then park and shut off. but then over the weekend when i drive around a lot it will jump up to 18 mpg because of the hours of idling rolling off.

GTyankee 10-19-2012 12:38 AM

the only way to get a true reading at the gas pump, is to pump your gas at 60 degrees F

that is why the people that know, say to fill your tank in the early morning, during the hotter months

Out here, the Dept. of Agriculture checks the gas pumps at regular intervals & also on random dates & times
Any pump that is found to pump more or less than the proper measured gallon, is RED TAGGED, they have a correction chart for temperatures

here is the chart

brad12kx 10-19-2012 01:55 AM

Adding to what davidgcet said; the line on your 5 gal gas can is very, very, very approximate and is only usable when determining how much oil to add if you are mixing fuel for a 2 cycle engine. The Weights and Measures department that tests these pumps uses a device called a prover which has a custom designed narrow neck with a sight glass that after being properly calibrated is very accurate but pretty much useless as a portable gas can. :)

GTyankee also brings some great information to the table (Thanks for posting the temperature correction chart! :smileup:) that has an affect on how much fuel you purchased. In Canada your fuel purchases are corrected to 15deg Celsius and in the US to 60 deg Fahrenheit (Basically the same temperature)

What this means is the expansion of fuel with temperature is great enough to take into consideration. This also ensures the issues can be caught using the daily (or more often) tank dips which are also temperature corrected. If the dips do not agree with the pumps, then either the pump is wrong or the tank is leaking.

I have heard the claims that yo should buy your fuel early in the morning when it's cool to get the most fuel. Although this may have been true many years ago, the temperature correction system makes this no longer effective.

Another consideration; if the station has in-ground tanks, the temperature of the fuel is very stable, and remarkably close to 60 deg F. The time of greatest fluctuation is when the tanks are filled. On a hot day, the tanker truck is pouring in some pretty hot fuel that may take a day to stabilize. Conversely, in the winter (especially in the northern states and Canada) the fuel is pretty cold when poured into the tanks. These are the times the correction at the pump is the greatest.

In the end you will pay the same price for the amount of 'energy' you bought, which in the case of gasoline is measured in BTU.

The EVIC in your truck is a 'guideline' and will never pass a calibration test for a 'legal for trade' liquid meter. In fact, it is not capable of even getting close as it uses the gas gauge as an input device to know how much fuel is in the tank. I know that most people know their vehicle well enough to know if the top or bottom half is bigger. In calculating mpg accurately, the top and bottom halves MUST be exactly the same. Although our fuel gauges are much more accurate then they used to be, they are still just a general guideline.

Hope this helps. :)

kman2424 10-19-2012 12:41 PM

Thanks for all your information i didn't realized how much the temperature of the gas effects its volume. It just bothers me that the EVIC is more accurate but whateves. Yea i just did assume though the gas can was very precise. And my EVIC is reading within +-10% so thats not too bad i guess.

Thanks for all your insightful responses :smileup:

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