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Discussion Starter · #1 ·
I just took two long legs of a journey over the past weekend. I wanted to determine the accuracy of the trip computer's calculations of the Average MPG. I carefully calculated it using actual fuel and mileage. Here are the results:

Trip Actual MPG Trip computer MPG Error
Leg1 41.5 40.8 -1.6%
Leg2 37.8 41.1 8.8%

My guess is that it grabs the current MPG periodically and bases the average on that. It's obviously NOT based on any real-time calculation or the results would be similar. Leg1 was a trip up the northern New York State, Leg2 was the return trip sot the terrain was almost identical.

An additional note, this is a new car, it now has around 1,600 miles on it.

I will repeat this test after the car breaks in.
 

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inconsistent fillup levels can really throw off accuracy, especially with small tanks.
example a half gallon top up difference in a ten gallon tank, thats five per cent right there.
thus more data is more meaningful:)
i look forward to your results after engine is broken in, averaging three or more consecutive fillups:) the 1.6 really rocks the mpgs!
edit my Altima calculator is also consistently optimistic
 

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Discussion Starter · #4 ·
By any chance were you in the VT area during that trip?
I drove from western Mass to Lake Tupper. I90, I87, NY28, NY30. I87 runs parallel to the VT border, I listened to a Rutland radio station, so yeah I was close to VT.

My mileage was better on NY28 and NY30 than on the interstates. Probably because I was limited to under 60 on those roads.

The car doesn't coast down long hills very well, it tends to slow down on hills that my old Hyundai accelerates on. Maybe that will change as the car breaks in.
 

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Discussion Starter · #5 ·
inconsistent fillup levels can really throw off accuracy, especially with small tanks.
example a half gallon top up difference in a ten gallon tank, thats five per cent right there.
I try to use a consistent fill method. Fill until click, fill some more until click, then fill to a third click. After the first click I reduce the fill speed to as slow as the pump will handle.

It's not a good idea to overfill but I normally drive for a bit after every fill.

If you average the figures it works out to measured 39.62MPG and reported 40.96MPG or a 3.35% error. That's not as bad as I expected.
 

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Discussion Starter · #6 ·
I'm certain the Average MPG calculator works as I thought. Tonight my reading was 40.4 after driving 335 miles, I drove 12 miles and got the average up to 40.9. That's a little hard to believe unless it picked up a couple times when I was coasting down hill.
 

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Discussion Starter · #7 ·
The mpg computer is not so bad in the long run. I've been keeping accurate records for about 1500 miles and five refills. The mpg error is only 1.68%, close enough for gummit work.
 

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I am having the same problem except it seems to be getting worse. My trips have been as follows. 327miles/9.32gallons 318miles/9.30gallons 328miles/9.46gallons while mileage calculated versus computer have been as follows. 35.07actual/38.3computer 34.19/38.1 and 34.67/41.5. I am currently at 1000 miles. It just appears that the trip computer is getting more off each fillup. Any ideas?
 

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Yeah the trip computer MPG can be off, the couple times I've filled up it was showing average of 42 MPG but really I'm getting 38 MPG I even reset the mpg avg readout after I fill up. I just go by the ODO miles and gallons I filled up, easier if you have a MPG calculator app for your phone.

Don't get me wrong I love having the readout but sometimes you just have to calculate it the old way. There are probably some parameters in the way it calculates we are not aware of so it just gives estimates instead of a true figure.
 

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For all those looking to smart phone solutions for keeping track of average mpg..

I would recommend "FuelLog" for android users.

All u have to do is enter ur total odo reading each fill
and the
total fuel cost + fuel quantity (or unit fuel cost)

easy to use (Y)
 

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Resetting Avg

So the computer on my 16 Note SV has been working fairly well (says 40 mpg & actual math says between 38 and 39). So I decided to zero it out and start over. Now all I get are strange numbers in the forties and fifties.

Does anyone have any idea how to get it back in sync?
 

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It has an instant read out then a cumulative readout since last time it was reset. When on the cumulative read out press and hold the button and it will zero out.

I think the issue is its just a straight average vs cumulative or vice versa so its not accurate. Having worked in a call center and being measured to a tenth of a decimal I know all the ways to play with numbers and stats. >:D

Plus a lot of GPS do not take in account elevation changes. So if you transverse 11 thousand feet on a mile long route looking at it vertically on a map it doesnt take in the extra mile you did in elevation change.

MPG is effective by everything. Winter vs summer formula of gas, ethanol content, temperature during fill up, etc.
 
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