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Old 03-09-2014, 10:37 AM   #1
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Why is the MPG computer not reliable?

I have read many posts that say the on board computer that displays MPG is not accurate. Can anyone explain this? How far off is it and why do they have it if it is wrong? I have a 2007 40ft Phaeton and it is impossible to fill my tanks to the same level each time (I could but it would take forever due to the foam). To me, not having the same fill each time would be more inaccurate to use than the computer.
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Old 03-09-2014, 10:50 AM   #2
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You have to remember it's the internet. Some people say things only because they're not looking you in the eye when they say it.

Most of those people claim that there is no real accurate way to read mileage, but if that's the case then the computer is just as accurate as anything.

The one thing they won't ever do is put a number on anything. They tell your computer is off, but never say how much (10%-20%-100%).

Someone on here had figured out their computer is off by 5%. That's the only one I've read about.

Sometimes when I check ours it'll show 1-2 mpg difference from hand to computer. Other times they're exactly the same.


So next time you're in a thread with somebody like that, tell them you were driving Newell and were get 20 mpg. See if you can get them to chip a key on the key board while they're typing how wrong you are.
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Old 03-09-2014, 11:31 AM   #3
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The computer provides a somewhat educated guess, and does not account for fuel use by the generator or Aqua Hot. Semi-accurate MPG is done over an extended period using a calculator.

But, what does it matter? It is not something you can change other than possibly with your right foot, as long as all is mechanically well with your rig.

I look at the computer on occasion. I never use a calculator. I do keep things in perspective though.

If one rig gets 8MPG and the next gets 7, over 5000 miles at $4 per gallon, the difference is $357. Over 6 months it is about $60/month, over a year about $30. In perspective it is chump change.
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Old 03-09-2014, 11:39 AM   #4
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Quote:
Originally Posted by Pa Uon View Post
. . . it is impossible to fill my tanks to the same level each time (I could but it would take forever due to the foam). To me, not having the same fill each time would be more inaccurate to use than the computer.
There is no need to fill to the same level when you measure milage over a greater distance that requires multi fill ups. The only consideration is that the final tank be filled "as close as possible" to the level you started out with. To try to do MPG calculations on individual tanks will almost always yield an inaccurate result because of the many variables involved in what determines you MPG, ie wind, terrain, weight of your right foot etc etc. The better method is to do it over numerious fill ups and operating under varying conditions to get a semi-accurate reading.
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Old 03-09-2014, 11:42 AM   #5
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Two things people never stretch the truth about is how big the fish was and how good their fuel mileage is.
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Old 03-09-2014, 01:32 PM   #6
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can never understand why people ever think about gas mileage in a rv. when people ask me what kind of mileage I get I tell them I've honestly never checked but its somewhere around worse to none. you don't buy an rv to worry about miles per gallon. convenience costs, so just fill up and pay the bill
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Old 03-09-2014, 01:36 PM   #7
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Quote:
Originally Posted by Pa Uon View Post
I have read many posts that say the on board computer that displays MPG is not accurate. Can anyone explain this? How far off is it and why do they have it if it is wrong? I have a 2007 40ft Phaeton and it is impossible to fill my tanks to the same level each time (I could but it would take forever due to the foam). To me, not having the same fill each time would be more inaccurate to use than the computer.
The mileage computer reads a host of variables, all of which are affected by outside circumstances. The best you can hope for is a value +/- some parameter. Fill your tank to the first auto-shutoff, record your odometer, and continue your trip. As a general rule, if you're gross weight is somewhere around 100 lb per HP in a diesel, you'll probably get about 7.0 - 7.5 MPG, depending on other variables.
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Old 03-09-2014, 01:49 PM   #8
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can never understand why people ever think about gas mileage in a rv. when people ask me what kind of mileage I get I tell them I've honestly never checked but its somewhere around worse to none. you don't buy an rv to worry about miles per gallon. convenience costs, so just fill up and pay the bill
You check the mileage for the same reason that you check the oil pressure, tire pressure, air pressure, coolant level, batteries, lights, etc., etc. You're responsible for a large, complex machine that has your family in it, and the details can give you an indication of current health or impending problems.
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Old 03-09-2014, 01:58 PM   #9
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The mileage computer reads a host of variables, all of which are affected by outside circumstances. The best you can hope for is a value +/- some parameter. Fill your tank to the first auto-shutoff, record your odometer, and continue your trip. As a general rule, if you're gross weight is somewhere around 100 lb per HP, you'll probably get about 7.0 - 7.5 MPG, depending on other variables.
That is pretty close. my MH weighs 31,000#, with 350HP = 88# per HP, and my MPG averages around 7-7.5. Has been as low as 5, as high as 8.5. While we were switching from a 5er to MH, we discussed MPG as we averaged 10.5 with our Duramax dually towing a 15,000# 5er. The difference over 1,000 miles is around $152 in fuel. And the dually has a 300HP engine, that's 76 HP per hundred pounds. I think that difference is minor.
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Old 03-09-2014, 04:30 PM   #10
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There are several "innauccaries" on the display.. First on my motor home I have my choice of "instant" and "Average".. The instant, I very strongly suspect, is accurate within the device limits (it will show ---- or 000 if out of lmits)

The average.. Every once in a while the comptuer resets and it will show like 148MPG or some other outlandish number.. Give it about 10 miles and it's reasonable again.. I suspect that is very accurate as well, HOWEVER.....

The reason: The engine control computer is what is reporting MPG, it actually knows how far you have traveled (From speed and millage sensors) and how much fuel it has injected into the cylinders. IT knows how to divide to many places.. So it's accurate, at least as accurate as the operating system can be (I do NOT know it's bit level).

BUT it can NOT tell how much fuel is burned for non-engine uses. How much is lost to evaporation or by other means.

Plus, enough idle time will mess up it's calcualtions (Engine running, in park) since you are burning gallons but not racking up miles.
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Old 03-09-2014, 05:09 PM   #11
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I used to think my on board computer was off also. However, now in my calculations, I include fuel burned by the Aqua Hot (eight gallons out of the last tank), by the generator and while at idle (while parking, slides in/out, jacks up/down). Now the computer is right.
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Old 03-09-2014, 05:15 PM   #12
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Close counts. Accuracy us not needed. It's use is as a relative gauge. To adjust driving speed and behaviors, awareness of something gone awry, etc.
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Old 03-09-2014, 05:22 PM   #13
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My Scan Gauge II can be fine tuned to where it is pretty accurate. When I pull up to the pumps I check the gauge to see how much fuel I have used. I have it now set to where it is usually within a gallon of what I actually pump. A lot of generator use will upset the numbers as it only measures what goes thru engine.
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Old 03-09-2014, 06:45 PM   #14
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......The engine control computer is what is reporting MPG, it actually knows how far you have traveled (From speed and millage sensors) and how much fuel it has injected into the cylinders. IT knows how to divide to many places........BUT it can NOT tell how much fuel is burned for non-engine uses. How much is lost to evaporation or by other means......
I'm not sure if it reads temp, humidity, and baro pressure, which will affect fuel injection; perhaps it does. It doesn't read relative wind; it doesn't know how much tread is worn off the tires, which will affect odometer accuracy, etc., etc.
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