Need to understand the logic "high rail pressure=high mpg on the lie-o-meter"

[hd99fxr3]

OK so I understand that high rail pressure means the engine is not under load requiring huge amounts of fuel, add that to rpms, plus mph at any given time equates to xx.x mpg per the algorithem in the ECM.But high did it get 4mpg off? I've been zeroing it out at fill up and it seems to be doing better. I'll be checking it in the next day or so by hand calculation when I fill up. My hand calculations have shown so far that I'm getting just over 18mpg running 75-80mph and a bit over 14 when towing the goose neck 65-70mph, with some local running around thrown in to skew the numbers a bit. But still not bad IMO. But how can the lie-o-meter say I'm getting 18-22mpg when I'm getting 14-18? I've had the truck for 6 weeks and have not done any mods other than add a B&W GN hitch and get the Bed Liner sprayed in.Thoughts

[gassernomore]

If there is a difference in tire size from what was original on the truck it will skew the overhead readings.

[hd99fxr3]

No they are stock sizes, but you are right taller tires will skew mph slower and mpg higher.

[Wild and Free]

I have never had any faith on the overhead reading on any of my 2nd or my 05 and especially the diesels. They have never been anything but a range reading and not necessarily a close range from my experience of the last 3 2nd gens and my 3rd gen.But on another note the one in my 2010 Hemi is almost dead on as It has been within a few tenths most times I fill but it actually reads less than actual by a few tenths most times but overall really accurate.

[anoldbiker]

Found out on my 06, the lie-o-meter reads 2 mpg over the true hand calculation mpg. So I use it only to see if I'm improving or screwing up my mpg on runs. Been constant since new.

[hd99fxr3]

As I understand it, the lie o meter uses rail pressure, mph and rpm to determine MPG. So if the mph and rpms are right, you would need high rail pressure for the perceived load to achieve high mpg readings on the lie o meter.What could be causing high enough rail pressure to cause the meter to be off so far?

[Mopar1973Man]

I can give you this much... ScanGauge II uses 2 numbers. MPH and GPH to figure MPG. 55 MPH / 2.80 GPH = 19.6 MPGSo some how Dodge has made a overly complex algorithm for figure out GPH. From rail pressure, RPM, etc.

[AH64ID]

Your comments are the first I have ever heard rail pressure is used for the lie-o-meter. (you did call it a lie-o-meter, so I know you know it's false).

The ECM uses the mm3 injected to determine fuel flow. Based on load/rpm the engine selects from its tables to determine how much fuel it needs to inject, then determines pressure/timing/duration as well. The ECM says hey, were injecting 130mm3 of fuel right now, that's "x" GPH.

The lie-o-meter is off because of programming. Mine was accurate, ish, thru about 12K miles, then it got progressively worse, and no averages 17% off, but varies from 8% to 34% off based on something, maybe the moon phase?

You can't just say, hey mine is 2mpg off because the error changes, and is a percentage not a mpg value off.

[hd99fxr3]

I believe it was over on the CF forum, that I read that Rail pressure was used in the calculation. At the time my impression was that at any given time rail pressure, mph, etc, etc. would determine actual load on engine, where by high rail pressure for any given rpm would equate to light engine load, when the other measured parameters agreed.

Again it was what I took any from the post over there. It made sense that if the injectors arn't bleeding pressure off the rail, and boost is low, throttle is not open more than other paramiters would indicate, the engine load is light.

For my qualifier a simple example is needed. Because as I understand it, higher rpms=higher pressure produced by the CP3.

For conversations sake let's say ( mostly because I don't know the exact PSIs ).

At idle the CP3 produces 5k-9k depending on throttle position, warm or cold.

At 1500 rpm empty on flat land 9k-15k

At 2000 rpm same as above 15k-23k

In each of these instances a specific RP would be needed to maintain the speed and rpm, if determined by the other perameters to be correct then logic would dictate a specific GPH consumtion (at that moment in time). So as long as the rail pressure stayed consistant for that perceived load, the ECM would give XX.X MPG on the readout.

My understanding could be flawed, and I would be greatful if someone could point out my error.

[AH64ID]

As a 90% rule rail pressure increases with load, not decreases. Your understanding is correct, but your over simplifying it. There are far more factors than just pressure, you have timing/duration/# of events to deal with as well. But in as few words as possible, the overhead is off because they screwed up the programming of it.

[morgancrew]

Huh... that sucks