MPG readouts are simpily a caculated value from the vehicle's computers. Always have been!

 

The computer knows the specific volume of fuel called for in each injection event and it knows how many events have occurred over a specified time period. It is an easy calculation from there with any modern computer system.

I find that even when I am very precise about filling my tank to the exact same limit each time with the truck in the same position, the pump says that I put on average 3% more gallons into the tank than the display tells me I have used. At the same time my odometer reads low by 3% when checked against mile markers on the highway. In my case the extra miles equate to the extra gallons so I have always considered the economy meter to be highly accurate within 1% or less.

My 6.0 was not quite as close in accuracy as the 6.7 is, but it was always within 0.25 mpg every time I hand calculated. It isn't as much of an issue on these trucks as it was on the 6.0, but unless you spent 5 minutes topping off the 6.0 you could never know for sure that you were filling the tank consistently each time. I always wondered how accurate the people are that report the dash display to be wildly inaccurate.

 

That's awesome thanks!

Already signed up and app downloaded...

 

Good info, will check it out.

 

This is spot on. This is how the modern ECU keeps track of fuel use. What makes the 6.7's computer much more accurate than times before is that each of the 8 injectors are flow tested and matched with the ECU. Now that teach injector is flow tested, the ECU knows exactly how much fuel is pushed through the injector for a given pulse width. Before, manufactures took the advertised flow rate of the injector and multiplied it by the time the injector is open for a given pulse. We see lots of flow rate variation in the tests we run.

We must also keep in mind all of the sources of error. How much error is in the flow test measurement? How much error are fuel pumps at the station allowed to have? How much error is there in the ODO that we read? An of course, the largest area of variability is the user's ability to consistently fill the fuel tank. A long term average like Epic's is far more accurate than someone's hand calculations.

For the first time, I think the meter that's in your dash is probably more accurate than hand calculations. Modern emissions standards require engineers to keep track of all fuel that goes through the engine because a too rich or too lean condition can cause significant problems with the downstream emissions package. Could this be why the 6.4 has more issues?

Cummins and I'm pretty sure the rest of the industry flow matches their injectors as well. Its becoming an industry standard.

If and when injectors need replacing...the new flow rate of the injector will need to input into the truck's computer.

I would put 20 bucks on my hand calculations to be more of LOM than the dash display.

 

Good catch, Matt. I checked my odo and found it to be 4% slow which accounts for all but ~1.0% of the LOM error on my truck. Assuming that the error would likely have been closer to 5% when the tires were new, that pretty much explains it away.

Good catch!

 

Yes and this is pretty much what Glockin' Bob and 720Deere said and which I completely agree with. If this is, in fact, what the system does, it ought to be able to indicate with extreme accuracy the gallons of fuel used. Yet several people have indicated that they are putting in more gallons than are indicated by the system and that includes me.

I fill up at the same place, same pump, pointing in same direction, and do everything exactly the same way every time and every single time I put in more gallons than what they system says I used. The error is in the neighborhood of 5%. and it has never happened once that I put in less gallons than the system says I used.

So why is it counting wrong if it can count as accurately as we think it can?

I don't agree with your assertion that this system is so accurate now that I must be wrong with my observations or that the fuel pumps are that far out of calibration. No. I think something else is going on and there aren't many places to look for the error.

Either the system is not as sophisticated as we think in terms of precisely measuring fuel it is putting into the engine or it is not counting everything and if it is not counting everything what might it not be counting? I guess it is not counting regen fuel. Any takers on that theory?

 

I can't provide better scientific evidence than has already been posted, but, the mpg goes down significantly during an active regen so it must be counting something......

 

I would suggest trying another fuel pump next time you fill up to confirm your findings. It would be prudent to eliminate your usual fuel pump as a possible source of error.

 

Maybe they missed the flow test and match phase on the injectors or somehow keyed the wrong numbers?

You haven't had any injectors replaced either right?

Only thing I can think of why yours is off that much.

Interesting thread, I will report back on how accurate mine is as I just joined fuelly so I can track it better, and also pay more attention to total gals consumed and compare.

This info was not available on my cheby

Have I said what an awesome truck yet

 

Those of you with EDGE CTS monitors, Just go change the PID that the monitor is watching and you can see the fuel flow for the injectors. The Edge is only displaying what the computer is alread tracking. So yes the Ford FCIM is tracking how much fuel is used per injector pulse and just doing the math to give you a mileage number.

Mine is accurate enough that I don't worry about doing the math by hand. My fuelly below in the signature show my total miles/gallons used. So it is no longer affected by how well I topped of a tank. Since it is averaged over a 100+ fill ups.

I started teasing the cashers when Ihave to pre pay by looking at my trip before I go in and telling them I'm filling up and will need 32.5 gallons. And I'm usually with in a few 10ths. when I go back to pay.

 

Mine has been spot on. Data is in link in my sig although I don't have any pretty graphs like Epic....yet

 

I just fill it. It gets what it gets and it costs what it costs. I love my truck!

 

For some reason, that made me laugh. I do the same, don't even know what the current price is, doesn't really matter, when it gets down around 1/3 of a tank, I fill it.

 

Exactly. Fill, drive, repeat. Enjoy!