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Old 07-07-2008, 08:50 AM   #1
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Proper fill-up?

Just as the title implies. When you are filling your car up, do you stop when it kicks off the first shuts off, or do you top it off? Mind you, I do not have a scangauge, I just use the simple math to figure my MPG and keep it in a spreadsheet. I ask this right now particularly because I filled up on Friday or Saturday, and at the gallons when the pump first kicked off I was right around 40MPG, but I topped it off and brought it down to 37.2.

Up until this point, I usually always top it off and I run the tank pretty low. With a 13 gallon tank, I am normally refilling with at least 11 gallons.

I would like to hear from anyone regarding this. But I am very intersted in hearing from those who use a scangauge to see how accurate their gauge is with respect to gallons used and what it actually took to fill it back up to when the pump first kicks off.
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Old 07-07-2008, 09:08 AM   #2
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It's tough to get consistent, accurate fills with topping off, so I stop at the first shut-off.
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Old 07-07-2008, 09:57 AM   #3
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The most important thing is to be consistent. If you always top-off or always stop at the first shut-off, you should be OK. If you alternate between them, then your economy will go up and down as the amount of fuel required to "fill" the tank changes.

Note that there will be some differences anyway. If you always fill at the same station, using the same pump, you'll get minimal differences. If you fill at different stations all the time, you will get larger differences.

No matter what you do, looking at long terms trends (5 or more fill-ups in a row) will even out these differences and give you a true picture of your overall economy.
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Old 07-07-2008, 10:01 AM   #4
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some people will say to use the same station and the same pump and fill up the same time of day. that is the best way to do it.

at the same time, if you are like me, you need to fill up in different places and different cities at different times of the day. I use the "first shut-off" method and have had good results. the best thing is an overall average. that way the err is averaged out. if you have a difference over one tank, lets say 13 gallons, that difference will be much smaller over 130 gallons or the equivalent of 10 tanks of gas. in the same respect, the difference could be huge with only 3 gallons. I have seen people claims great mpg using just a few gallons of gas.

that is the beauty of the gas log. it will average it for you. most people (not all) have fluctuations in their gas logs. one tank will look great and the next won't if you average out the two tanks, you will get a better idea of what the true mileage is.

I have a scangauge but still use the calculation method because of accuracy. the scangauge calculates MPG using air flow (this is something new to me too) so I have had it where my calculated mpg was better and worse (not at the same time) than what the scangauge was reading. I like to think of the scangauge as a good indicator. I still use the trip odo and gallons used method.

*edit* crap, slow to post didn't see the post before mine
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Old 07-07-2008, 10:07 AM   #5
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Here's the way I do it.
Fast set to click off
Wait 5 seconds
Slow set to click off
Wait 5 seconds
Fill to next dime
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Old 07-07-2008, 10:12 AM   #6
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I always stop at the first click. Overfilling the tank can cause problems with the vapor recovery system on the pump, and recirculate gasoline which will make your mileage appear lower than it actually is. Overfilling the tank can also cause problems with the vapor system on the vehicle as well.
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Old 07-07-2008, 10:19 AM   #7
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I understand consistency. I realize that if I suddenly start going by the shut off and not top off that my results will look skewed for a bit since I've usually topped off in the past. And for the most part I do go to the same place for gas. Not always the same pump though. I know here in NY anyway that gas pumps are tested every few years for accuracy so I am not worried about inconsistencies between different pumps or even stations. It should not be that big of a concern with such a small amount of fuel in relation to how much they dispense overall.

I've seen what you are referring to about some with just small fills and showing big numbers. The problem lies with margin of error on the calculations. This margin is much smaller when running the tank down and filling it all the way back up, as opposed to running down a couple gallons and then topping off again. Consider that many vehicles can squeeze another 1/4 gallon, if not more, after the pump shuts off. A 1/4 gallon difference makes a big difference if you're talking only 2 gallons (+/- 12.5%), whereas it is not so significant when we are talking 10 gallons (+/- 2.5%). For that reason is why I believe
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Old 07-07-2008, 10:47 AM   #8
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Quote:
Originally Posted by froggy81500 View Post
gas pumps are tested every few years for accuracy so I am not worried about inconsistencies between different pumps or even stations.
That only tests that they measure the pumped amount of gas accurately, not that they shut off at the same point.

For accurate and consistent MPG calculations, you want it to shut off at the same point so it reaches the same level in your vehicle every time. That means the best accuracy is on the same pump, same side, facing the same direction every time.
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Old 07-07-2008, 11:22 AM   #9
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Quote:
Originally Posted by theholycow View Post
For accurate and consistent MPG calculations, you want it to shut off at the same point so it reaches the same level in your vehicle every time. That means the best accuracy is on the same pump, same side, facing the same direction every time.
It's the Feng Shui method of gas logging!



OTOH, you are absolutely right about the shutoff variation. The only description I've heard of the test for pump accuracy involved pumping exactly 5.000 gallons of gas (according to the pump) and measuring it some other way (I don't remember what, maybe just a demarcated container). It doesn't use the auto-shutoff, which will still accurately measure how much was added to the tank, but won't accurately measure how much was used.
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Old 07-07-2008, 11:43 AM   #10
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Quote:
Originally Posted by thornburg View Post
It's the Feng Shui method of gas logging!



OTOH, you are absolutely right about the shutoff variation. The only description I've heard of the test for pump accuracy involved pumping exactly 5.000 gallons of gas (according to the pump) and measuring it some other way (I don't remember what, maybe just a demarcated container). It doesn't use the auto-shutoff, which will still accurately measure how much was added to the tank, but won't accurately measure how much was used.
Yeah, they use a special calibrated container to measure the gas pumped. The pump needs to read 5 gallons when the container is full.

-Jay
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