How Accurate is the on board MPG estimation?

I have a 2013 Accent GLS and, like my son's old Elentra GT, this has a running average MPG calculation. It definitely has some correlation to reality since I can influence it one way or the other if I either am
heavy on the gas or try to speed up down hills and slow down up hills - accelerating mildly.
I was wondering how accurate it tends to be over a few hundred miles. If I am very careful and do more steady state driving with not a lot of traffic and only some stop and go I can coax it to over 40 MPG. Careful driving in town (not huge city) with some traffic drops it down to 36 MPG. That includes fairly slow acceleration. My wife says if she had known it was so easy to get me to drive gently/slow down she would have found a car with that ages ago.
Funny thing is that in ECO mode my mileage drops - probably because my light-footed driving is conflicting with the fuel saving algorithms. During test drives the ECO mode seemed to cut down on my wife's ability to accelerate hard, etc. It shifted earlier, etc.
KW
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Based on what I've seen on trip computers and comparisons to what customers tell me, the trip computers are rather accurate. I use them regularly to test fuel economy complaints.
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On 3/21/2013 11:29 AM, hyundaitech wrote:

tell me, the trip computers are rather accurate. I use them regularly to test fuel economy complaints.

Thanks! Well considering that this last week I averaged 41.1 MPG (last tank full) on tollway going 75 mph (apx) and will be reimbursed for the business trip at set rate, I am quite happy!
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KWW wrote:

Quite accurate on all kinds of cars. I rent a car every two weeks for business trips and write down odo mileage at every fill up. The cars mpg calculator is about the same as my own calculations.
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wrote:

I had the mpg trip meter on both my 2010 and 2013 Sonata. The reading I get is consistently higher than the actual calculated mileage.
Closest I ever came was 0.5 mpg. Most were about 2 to 3 mpg short of what the meter said.
Here is a sample of what I have recorded: Computer    Actual 27.1        22.87 26.7        24.93 26.3        24.62
In every case, I filled the tank myself at the same gas station and they were sequential tanks so if one was off, the next would correct.
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