Fooled by the MPG-ometer - Page 2 - JeepForum.com

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post #16 of 39 Old 12-19-2013, 11:48 PM
ncb
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It's not accurate and never will be %100. Way too many variables.

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post #17 of 39 Old 12-20-2013, 12:01 AM
06bluebeast
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I'm not sure if coincidence or not, but mine used to be 1-2 full mpg off. I installed a bully dog and now my dash is almost dead nuts. The bully dog is usually low by 1-2 mpgs.
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post #18 of 39 Old 12-20-2013, 12:03 AM
SubAtomicGenius
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I especially like how some people expect/demand NASA-like precision for domestic vehicle fuel consumption meters.

In most cases, a calculator will always be more accurate than fancy overseas electronic doo-dads that they put in automobiles. You can get one for $1 at the dollar store (though I'd say that they're also made overseas, too).
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post #19 of 39 Old 12-20-2013, 03:31 AM Thread Starter
Yendor
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Huh?
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post #20 of 39 Old 12-20-2013, 06:07 AM
SubAtomicGenius
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Yeah, that's what I thought.

1. Fill fresh tank
2. Reset odometer
3. Drive
4. Fill tank
5. Divide miles on odometer by gallons filled using a calculator
6. Observe MPGs
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post #21 of 39 Old 12-20-2013, 06:11 AM
Rammer500
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I'd bet the MPG readout is more accurate then most give it credit for. The ECM knows exactly how much fuel is being used by how many mili-seconds the injectors are open. Injector flow rate, injectors open time and distance traveled is just simple math for a computer. The amount of fuel being put back in and speedometer accuracy are more likely the issue.
For example, New tires travel fewer revolutions per mile than worn out ones do, so the speedometer accuracy changes as your tires wear. Tires with 19/32" tread worn down to 3/32" of tread are now 1" smaller in diameter.

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post #22 of 39 Old 12-20-2013, 09:38 AM
Tbasham
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Has anyone ever noticed if jeep MPG readout is more accurate depending what season it is? Depending on what type of sensors used, temperature can greatly effect the error in what the sensor reads out. Really hot summer days or really cold winter days could effect the sensor performance. During the summer months i found that my jeep readout was roughly .7 mpg less then what I hand calculated.
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post #23 of 39 Old 12-20-2013, 10:02 AM
number40Fan
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All the computer needs to do is calculate the amount of fuel used by the amount of injector open time. The rest of the sensors on the engine are used to calculate injector open time.
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post #24 of 39 Old 12-20-2013, 10:34 AM
COLOUXJ
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Quote:
Originally Posted by number40Fan View Post
All the computer needs to do is calculate the amount of fuel used by the amount of injector open time. The rest of the sensors on the engine are used to calculate injector open time.
There is way more involved in calculating flow rates of fuel than the time that the injector is open.


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post #25 of 39 Old 12-20-2013, 10:44 AM
ronjenx
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Let's face it. No one here knows exactly what data the computer uses to get a miles per gallon reading. It's an extrapolation at best.
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post #26 of 39 Old 12-21-2013, 07:56 AM
aTX427
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Is your speed and distance calculated from the GPS or a speedometer gear like the older Wranglers? Could different tire sizes throw off the computer's mpg calculation?
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post #27 of 39 Old 12-21-2013, 10:17 AM
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The tire size plays the biggest part of the calculation. So yes, if you tire size is not corrected and accurate the system will not be accurate.

^^Jeff^^
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post #28 of 39 Old 12-21-2013, 01:36 PM
73azbronco
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It's an "ish" meter. I get 14MPG"ish". But when the lady down the street asks, I get 42MPG on freshly squeezed baby seal juice.
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post #29 of 39 Old 12-21-2013, 02:35 PM
number40Fan
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Quote:
Originally Posted by COLOUXJ View Post
There is way more involved in calculating flow rates of fuel than the time that the injector is open.
By way more, you must mean fuel pressure, battery voltage, latency, relation of the moon to the Pacific ocean.
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post #30 of 39 Old 12-21-2013, 03:09 PM
krawler510
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My Mazda is direct injection and you'd think that could measure mpg accurately but it's usually 1-2 mpg optimistic. Unless it's pure highway, in which case it's very accurate.

'The early bird may get the worm, but the second mouse gets the cheese.'
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