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Anyone compared actual MPG (via math. tank top off to tank top off) to what the computer says on a run.

Would think with the precise metering the computer has to do for direct injection it is very accurate but has anyone verified the old way.
 

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I have twice. Math method is slightly lower,(not as good) but not off by much at all.
 

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i still habitually reset the trip meter A everytime i fill the tank for just that reason... but then i get lazy and look at the computers average at the next fill up. ill have to try this one time. id be interested to know also
 

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On my last tank, the computer said I was at exactly 31.0 mpg, but the actual was 29.3 mpg.
But, I only have 850 miles on my car so far, and that was the second fill up since buying the car. I imagine it may be closer as the engine breaks in.
 

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I have an app on my iphone I am tracking with.

Computer says:
27 mpg
27.9 mpg
34 mpg

Math says:
26.8 mpg
25.6 mpg
31.7 mpg

It is off, but not alot. I need to fill up on friday and will update the results.
 

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I guess my question would be, how does anyone know that they are filling up the tank to the same level? Just because the pump kicks off doesn't mean that it's exactly the same level everytime. If the computer says you are getting 30.0 mpg and when you fill up,you put 1 CUP less gas than the last time you filled up it would vary the numbers by 1.875mpg. I think unless you go the extreme and weigh the fuel to be positive you are getting the same amount everytime there ARE going to be differences.

Scotty B
 

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QUOTE (illby1 @ Jul 29 2010, 06:01 AM) index.php?act=findpost&pid=345815
I guess my question would be, how does anyone know that they are filling up the tank to the same level? Just because the pump kicks off doesn't mean that it's exactly the same level everytime. If the computer says you are getting 30.0 mpg and when you fill up,you put 1 CUP less gas than the last time you filled up it would vary the numbers by 1.875mpg. I think unless you go the extreme and weigh the fuel to be positive you are getting the same amount everytime there ARE going to be differences.

Scotty B
You are right, you dont really know that you are leaving the gas station with the EXACT amount of gas, its just an estimate. For the most accuracy, Ive always found that its best to go to the same gas station and try to get the same pump each time you do it and make sure you follow the same filling technique. LIke, if you stop after the first click, or do you go to a second or third click? Ive always liked going to 3 clicks when I fill up and I do that everywhere I go.

And I think your calculation is off. There are 16 cups in a gallon (8oz vs 128 oz). So lets say you went 250 miles and used 10 gallons of gas, that would be 25 MPG. If you went 250 miles and used 10.0625 gallons, that would be 24.84, a difference of 0.16 MPG. The difference is negligible.
 

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QUOTE (Mike Eckman @ Jul 29 2010, 07:54 AM) index.php?act=findpost&pid=345828
You are right, you dont really know that you are leaving the gas station with the EXACT amount of gas, its just an estimate. For the most accuracy, Ive always found that its best to go to the same gas station and try to get the same pump each time you do it and make sure you follow the same filling technique. LIke, if you stop after the first click, or do you go to a second or third click? Ive always liked going to 3 clicks when I fill up and I do that everywhere I go.

And I think your calculation is off. There are 16 cups in a gallon (8oz vs 128 oz). So lets say you went 250 miles and used 10 gallons of gas, that would be 25 MPG. If you went 250 miles and used 10.0625 gallons, that would be 24.84, a difference of 0.16 MPG. The difference is negligible.
I see what you are saying, but wouldn't you go by the numbers on the pump? If it says you put in 17.5 gallons, you have most likely put in 17.5 gallons. Wouldn't you just divide the number of miles driven by the number of gallons you put in the car? The amount of fuel in the tank should not matter. Just a newbie opinion... :whistling:
 

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QUOTE (illby1 @ Jul 29 2010, 06:01 AM) index.php?act=findpost&pid=345815
I guess my question would be, how does anyone know that they are filling up the tank to the same level? Just because the pump kicks off doesn't mean that it's exactly the same level everytime. If the computer says you are getting 30.0 mpg and when you fill up,you put 1 CUP less gas than the last time you filled up it would vary the numbers by 1.875mpg. I think unless you go the extreme and weigh the fuel to be positive you are getting the same amount everytime there ARE going to be differences.

Scotty B
I use the same pump at my favorite local gas station the majority of the time and set the flow on its slowest setting after seating the nozzle properly. I let the pump click off on its own and when that happens, I'm done. I use a credit card and am unconcerned whether the purchase ends at a round dollar amount or not.

I suppose I might get a tablespoon more or less here and there by letting the pump click off on its own, but I just can't put the act of getting gas under such scrutiny. I feel very confident I am getting consistent results.

(edited to add last sentence)
 

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You can see by my fuelly account that my average is around 30.9. I typically see 32-33 on the computer at the end of my tank. So I would say the car is off by 1-2 mpg or 3-6%. Not bad and I still enjoy seeing a overall average of 30+!
 

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QUOTE (azballer @ Jul 29 2010, 09:17 AM) index.php?act=findpost&pid=345854
I see what you are saying, but wouldn't you go by the numbers on the pump? If it says you put in 17.5 gallons, you have most likely put in 17.5 gallons. Wouldn't you just divide the number of miles driven by the number of gallons you put in the car? The amount of fuel in the tank should not matter. Just a newbie opinion... :whistling:
You have it correct. You go off what the pump says when you fill it up and just divide the total miles traveled on your trip odometer by how much gas you put in.

You dont always need to wait til your tank is completely empty. What I do is just pick a gas station I like by my house, and when I get somewhere under half a tank, I top it back off. That way I can be sure I go to the same station as often as possible and can get consistent results.
 

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QUOTE (illby1 @ Jul 29 2010, 07:01 AM) index.php?act=findpost&pid=345815
I guess my question would be, how does anyone know that they are filling up the tank to the same level? Just because the pump kicks off doesn't mean that it's exactly the same level everytime. If the computer says you are getting 30.0 mpg and when you fill up,you put 1 CUP less gas than the last time you filled up it would vary the numbers by 1.875mpg. I think unless you go the extreme and weigh the fuel to be positive you are getting the same amount everytime there ARE going to be differences.

Scotty B
I guess the math you are applying here is different that the regular one. If you fill the tank to 18 gallons or 18 gallon plus a cup does not make much of a difference at all. Anywas my computer shows 1 mile more per gallon than the math, so it is close enough. I have 3300 miles and I am giving up on calculating these numbers because the Sonata is an excellent car and it depends on me how many gallons per mile it gets because I am pushing the pedal.
 

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I have fuelly as you can see in my sig. My car calc showed my last mgp at like 29 ish, and i think fuelly was around 27.7. not too bad. especially considering that it's not easy to determine the ammount of fuel used in relation to the distance traveled. The gas tank doesn't fill up the same way, and aparently the temp can determine the volume of the gas (pv=nrt) from chemistry. I recall someone stating it's better to fill up at x point because you get more liquid volume, and when it's hot the gas (state) volme is larger so you get more "bang" for the buck. but i'm no chemist.

either way, i've found out that it's pretty close, not enough to completely worry about.
 

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QUOTE (illby1 @ Jul 29 2010, 04:01 AM) index.php?act=findpost&pid=345815
I guess my question would be, how does anyone know that they are filling up the tank to the same level? Just because the pump kicks off doesn't mean that it's exactly the same level everytime. If the computer says you are getting 30.0 mpg and when you fill up,you put 1 CUP less gas than the last time you filled up it would vary the numbers by 1.875mpg. I think unless you go the extreme and weigh the fuel to be positive you are getting the same amount everytime there ARE going to be differences.

Scotty B
Considering there are 16 cups per gallon, I don't think 1 cup is going to make 1.875 mpg difference on a fillup. More like .1 mpg in a 14 gallon fillup. Even if it was off a quart, it only changes by .52 mpg, if my (potentially flawed) math is correct.
 

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My math might be off I guess. I just used an example of 30mpg and there are 16 cups per gallon, I just divided 30 by 16 to get the 1.875mpg difference.

In the end all I know is that this car is getting about 10-15mpg MORE than the last car I was driving..... Which is nice!!

Scotty B
 

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Have about 7,000 on my Sontata Limited now. Actual calculated MPG for me is about 1.5 less than what the car estimates for me. A little disappointing that it is off by that much (not always, but more often than not), but no biggie.

I love the Sontata and this doesn't deter from that. Very pleased with the gas mileage to date.
 

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QUOTE (psuath @ Jul 29 2010, 11:30 AM) index.php?act=findpost&pid=345897
I have fuelly as you can see in my sig. My car calc showed my last mgp at like 29 ish, and i think fuelly was around 27.7. not too bad. especially considering that it's not easy to determine the ammount of fuel used in relation to the distance traveled. The gas tank doesn't fill up the same way, and aparently the temp can determine the volume of the gas (pv=nrt) from chemistry. I recall someone stating it's better to fill up at x point because you get more liquid volume, and when it's hot the gas (state) volme is larger so you get more "bang" for the buck. but i'm no chemist.

either way, i've found out that it's pretty close, not enough to completely worry about.
Actually, it's best to fill up first thing in the morning. That's when the fuel is at it's coldest, and therefore most dense. As a result you get more fuel by volume. As for efficiency, again, colder is better. The air is more dense and therefore more oxygen by volume. More oxygen = hotter burning = more power and better efficiency.

Reference: What is the best time of day to buy gas?

As for calculating milage, it doesn't matter how much you add, however the more gas you use and the further you drive, the more accurate your calculation will be. All you do is divide the miles driven by the gallons added. It's that simple. Just be sure to reset the trip meter every time you fill up.

Reference, take your pick: How to calculate MPG
 

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There was an old thread on this...but it's pretty much the same thing. I get between 1.5 to 2 mpg less in "real life" compared to what the computer says. I have 4080ish miles, just got an oil change. Hopefully my mpg goes up (been kinda disappointed with the 26 mpg with maybe 60% highway).
 

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The three times I have checked they were within .3 MPG and the last tank was reported lower than actually measured. Each time was at the conclusion of a trip. The reason i mention this is my wife's car seems to overestimtate the MPG on trips but underestimate around town. I haven't compared for a tank of in town driving yet.
 
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