Wild MPG instant reading - Page 2 - Ram 1500 Diesel Forum
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post #11 of 40 Old 11-27-2014, 03:00 PM Thread Starter
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Took the truck out for a 15 mile drive on two lane side roads. Seemed okay today but think I will take in next week and have them take look and see if there were any codes. There has not been any CEL, will post what I find out. ��
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post #12 of 40 Old 11-27-2014, 05:01 PM
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Good call on taking it to the dealer
keep us posted on what they report back.
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post #13 of 40 Old 11-27-2014, 11:35 PM
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1st no manifold vacuum on a diesel.
2nd, MPG is calculated using a number of factors (Probably forgot a few) including speed, engine load, throttle position, engine temp, etc. Don't reset for awhile and it should average settle down. On the big Rams 2500/3500 diesels we refer to these as lie-o-meters for a reason. The only mpg numbers we believe are hand calculated. Anything from the display is BS. They might get close but that's about it.

Bob
Current; '04.5, 2500HO,Auto,QCLB,4x4,CAI,4" TB,Smarty Jr,AD II 100,Flex-A-Lite fans, homebrew headache rack, "wedge" cap, rear bumper'03, 3500,QCLB,4x4,Dually,RIP
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post #14 of 40 Old 11-29-2014, 08:28 PM Thread Starter
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1st no manifold vacuum on a diesel.
2nd, MPG is calculated using a number of factors (Probably forgot a few) including speed, engine load, throttle position, engine temp, etc. Don't reset for awhile and it should average settle down. On the big Rams 2500/3500 diesels we refer to these as lie-o-meters for a reason. The only mpg numbers we believe are hand calculated. Anything from the display is BS. They might get close but that's about it.
I know what you mean about lie o meters as that is what I felt about those figures on my other Rams or Fords.
Mine has settled down with no more fluctuations up or down, This took place in a 15 to 20 mile stretch of I 5 four lane highway of bumper too bumper traffic and was a major pain in you know where. But still plan to take it to the dealer as had a CEL which I have no idea what it was four. Ray
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post #15 of 40 Old 11-29-2014, 11:33 PM
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That's the best way to go about it, i know way too many people that solely depend on the lie-o-meter.
best to get real world stats from other owners and even doing the math yourself.
then there are sites like fuelly to check
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post #16 of 40 Old 11-30-2014, 05:58 PM
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I'm guessing the ECM counts the number and duration of injections and calculates overall fuel flow from that as well as estimated rail pressure. Once flow is known, MPG = Speed(miles/sec) / Flow(gallons/sec).
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post #17 of 40 Old 11-30-2014, 06:37 PM
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I'm guessing the ECM counts the number and duration of injections and calculates overall fuel flow from that as well as estimated rail pressure. Once flow is known, MPG = Speed(miles/sec) / Flow(gallons/sec).
rpm, barometric pressure, ambient temperature, engine temp., vehicle speed, estimated fuel flow @ ?? RP (RP varies with with rpm/load, ECM can't calculate exact fuel flow it doesn't know what's going back to the tank/return line), boost pressure, transmission gear, TC locked/unlocked, throttle position. (missing a few I'm sure) as you can see with all these parameters the mpg display "might" vary just a touch. That is why the ONLY real MPG numbers are hand calced. distance traveled / gallons used. The less the display is reset the more consistent/stable "realistic" the display will become to calced mpg. When "tuners" come out the numbers that you will see reported will be all over the board. Tuners, depending on how they interact with the factory ECM and what tune level they are operated on, will give really wild extremes on the display.

Bob
Current; '04.5, 2500HO,Auto,QCLB,4x4,CAI,4" TB,Smarty Jr,AD II 100,Flex-A-Lite fans, homebrew headache rack, "wedge" cap, rear bumper'03, 3500,QCLB,4x4,Dually,RIP
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post #18 of 40 Old 11-30-2014, 06:41 PM
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Originally Posted by rdefayette View Post
rpm, barometric pressure, ambient temperature, engine temp., vehicle speed, estimated fuel flow @ ?? RP (RP varies with with rpm/load, ECM can't calculate exact fuel flow it doesn't know what's going back to the tank/return line), boost pressure, transmission gear, TC locked/unlocked, throttle position. (missing a few I'm sure) as you can see with all these parameters the mpg display "might" vary just a touch. That is why the ONLY real MPG numbers are hand calced. distance traveled / gallons used. The less the display is reset the more consistent/stable "realistic" the display will become to calced mpg. When "tuners" come out the numbers that you will see reported will be all over the board. Tuners, depending on how they interact with the factory ECM and what tune level they are operated on, will give really wild extremes on the display.

How does barometric pressure have any bearing on MPG? Once fuel flow is known, it doesn't matter what temperature anything is, it's all about speed and fuel consumption.
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post #19 of 40 Old 11-30-2014, 08:09 PM
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How does barometric pressure have any bearing on MPG? Once fuel flow is known, it doesn't matter what temperature anything is, it's all about speed and fuel consumption.
Barometric pressure is the measure of air density. ECM adjusts for that. for example: given the same weather conditions (NO tornadoes/hurricanes in the area) barometric pressure is lower in Denver (mile high city ring a bell/thinner air) than it is at sea level. System has to compensate for the amount of air ingested. Even with a turbo engine MAP (Manifold Absolute Pressure) sensor is still employed. More boost more fuel/less boost less fuel.

Bob
Current; '04.5, 2500HO,Auto,QCLB,4x4,CAI,4" TB,Smarty Jr,AD II 100,Flex-A-Lite fans, homebrew headache rack, "wedge" cap, rear bumper'03, 3500,QCLB,4x4,Dually,RIP
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post #20 of 40 Old 11-30-2014, 08:40 PM
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Barometric pressure is the measure of air density. ECM adjusts for that. for example: given the same weather conditions (NO tornadoes/hurricanes in the area) barometric pressure is lower in Denver (mile high city ring a bell/thinner air) than it is at sea level. System has to compensate for the amount of air ingested. Even with a turbo engine MAP (Manifold Absolute Pressure) sensor is still employed. More boost more fuel/less boost less fuel.


I really doubt this truck has a barometric pressure sensor on it. The truck does of course adjust for changes in O2 density (lower ambient pressure). But the sensor for adjusting for O2 fluctuations is the O2 sensor, not a barometric sensor.
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