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Trying to Get Lost
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Discussion Starter · #1 ·
I've been keeping track of the actual MPG vs. displayed MPG for a little while now, attempting to come up with an approximation of how far off it is.

Here's what I came up with so far, if anyone cares. :)

1. Actual: 16.2 Computer: 17.7 Difference: -1.5 (9.26%)
2. Actual: 17.8 Computer: 18.6 Difference: -0.8 (4.5%)
3. Actual: 15.5 Computer: 16.9 Difference: -1.4 (9.03%)
4. Actual: 18.1 Computer: 19.2 Difference: -1.1 (6.08%)
5. Actual: 16.9 Computer: 17.6 Difference: -0.7 (4.14%)
6. Actual: 19.0 Computer: 19.7 Difference: -0.7 (3.68%)
7. Actual: 20.7 Computer: 21.5 Difference: -0.8 (3.86%)

Based on that sample, the computer was overestimating by an average of exactly 1.00 mpg (5.79%), with highway tanks leaning toward higher accuracy (2, 6, and 7).
 

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The Bad Guy
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You're probably only as close as you are since I'm assuming you are also resetting the computer readout. Most people wouldn't be doing that which would increase the inaccuracy. Still, it's a pretty good approximation. The purpose is to improve driving habits which in turn improve MPG.
 

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Trying to Get Lost
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Discussion Starter · #5 ·
You're probably only as close as you are since I'm assuming you are also resetting the computer readout. Most people wouldn't be doing that which would increase the inaccuracy. Still, it's a pretty good approximation. The purpose is to improve driving habits which in turn improve MPG.
You're right, I'm resetting at each fill-up. Allowing it to continue calculating without resetting it would probably add significant additional error since it's including mileage that is not included in the "Actual" MPG.
 

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............The purpose is to improve driving habits which in turn improve MPG.
I dont worry about mpg in any of my vehicles....if I was worried about that, I wouldn't have gotten my TJ.......I don't drive to get better mpg either...it is what it is........:D
 

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Trying to Get Lost
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Discussion Starter · #7 ·
I dont worry about mpg in any of my vehicles....if I was worried about that, I wouldn't have gotten my TJ.......I don't drive to get better mpg either...it is what it is........:D
It's sort of a game for me, at least for a while. :) I like seeing how high I can get the numbers. I did it when I got my motorcycle too, until I got tired of recording all the data.
 

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It's sort of a game for me, at least for a while. :) I like seeing how high I can get the numbers. I did it when I got my motorcycle too, until I got tired of recording all the data.

Maybe I just dont want to know...that might scare me.....:eek:
 

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:D Yea, me to, I usually don't care about the mileage cause mine ain't a DD-

When it was fairly new On a trip to LA and back, I averaged about 19 mpg-

Now, maybe 15-but what the hey !!

:rofl::rofl: JIMBO
 

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Premium Member
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j42, did you factor in the accuracy of the gallons dispersed by the pump. I found here in Louisiana some of the gas pumps have not been calibrated and certified by the state for 3 years which leads me to question some of my mpg figures. Due to cutbacks the state cannot keep up with calibrations and certifications.
 

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Trying to Get Lost
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Discussion Starter · #11 ·
j42, did you factor in the accuracy of the gallons dispersed by the pump. I found here in Louisiana some of the gas pumps have not been calibrated and certified by the state for 3 years which leads me to question some of my mpg figures. Due to cutbacks the state cannot keep up with calibrations and certifications.
If there were inaccuracies in the pump measurements, they would be impossible to determine. And if they were there, I would say that they'd most likely be statistically insignificant anyway, especially when averaged over multiple readings like this. Additionally, three tanks were filled at a location 250 miles away from the other four.
 

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Good deal, I was just wondering because now that I think of it sometimes I would fill up my TJ and it would take 16 gallons and I would not be on empty yet. That leads me to believe the pumps were a couple of gallons off.
 

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I do my own stunts.
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I tracked the same thing the same way for a few months, with very similar results.
 

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Ugly Supporting Member
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j42, did you factor in the accuracy of the gallons dispersed by the pump. I found here in Louisiana some of the gas pumps have not been calibrated and certified by the state for 3 years which leads me to question some of my mpg figures. Due to cutbacks the state cannot keep up with calibrations and certifications.
Here in NM, we have the same issue with the state. After 2 years of not checking they finally got to me and found one of my pumps was off 6/100 of a gallon, which made me call in the service company to recalibrate that one and the one that was 4/100 off. 5/100 is the cut-off point in NM for recalibration. The numbers are probably not gonna be off that much in LA either.


I just finished a 3000+ mile road trip and found the on board system to be very close to what I was actually getting. The differences were in the 2 to 3 tenths of a gallon. I averaged around 14 m.p.g.
 

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I've been experiencing similar MPG data results, the onboard computer always tells me I'm getting better than what I record on my phone app.

Maybe it's my imagination, but it seems when I idle alot/stoplights, or I'm in alot of stop and go city traffic, the onboard computer says I get better MPG than what I calculate after a fill up.

If I'm constantly moving with very few stops (i.e., freeway), the MPG data seems more accurate. This leads me to believe that MPG is only being calculated by the onboard computer when the vehicle is in motion -- could this be true? Seems pretty lame if this is the case. IMHO the data should be calculated as long as fuel is being burned!

Tony
 

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It continues to read when you are stopped. Reset it drive a couple minutes and than stop and watch it fall. I reset mine when I was moving once and continued to drive for about 8 miles at 60 totally flat ground and was reading 26.2 when I got to where I stopped. Idiling kills your mpg, like this morning when it was 18 at my house and I let it warm up about 10 minutes. Lol
 

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I've been keeping track of the actual MPG vs. displayed MPG for a little while now, attempting to come up with an approximation of how far off it is.

Here's what I came up with so far, if anyone cares. :)

1. Actual: 16.2 Computer: 17.7 Difference: -1.5 (9.26%)
2. Actual: 17.8 Computer: 18.6 Difference: -0.8 (4.5%)
3. Actual: 15.5 Computer: 16.9 Difference: -1.4 (9.03%)
4. Actual: 18.1 Computer: 19.2 Difference: -1.1 (6.08%)
5. Actual: 16.9 Computer: 17.6 Difference: -0.7 (4.14%)
6. Actual: 19.0 Computer: 19.7 Difference: -0.7 (3.68%)
7. Actual: 20.7 Computer: 21.5 Difference: -0.8 (3.86%)

Based on that sample, the computer was overestimating by an average of exactly 1.00 mpg (5.79%), with highway tanks leaning toward higher accuracy (2, 6, and 7).
I noticed on my stock tires my readings were almost exactly like Jeff's, off by about one MPG on the high end. Now, since I swithed tires, goodyear wrangler DT's, I noticed I am getter better gas milage than the computer is saying and I always reset and always do the math from the amount I put in my tank and the miles driven on that tank.
I am actually very happy with my MPG in this Jeep, better than my TJ.
 
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