Lie-o-meter
Moderator
Joined: Jul 2011
Posts: 6,509
Likes: 658
From: Sioux Falls Area
That is quite a leap of faith in the on board computer.
So far I have purchased 19,194.82 gallons of fuel for my truck. At the time of that purchase the odometer read 298,765 miles. I hand calculate the lifetime fuel economy to be in the 15.5 mpg range ( includes any towing ).
fair enough, what does your readout display? While i applaud your monumental display of attention to detail, to me an overall lifetime average fuel mileage is not very useful. particularly if you are going to assume that anything that deviates from that overall average is wrong.
i'm more interested in trip-trip or tank-tank averages. as in, what did i get pulling my boat to the lake, or going to the hills with the camper. that type of thing. which is prone to some error. using hand calcs to indentify
My wife has an identical truck except it is a Lariat version. The overhead computer says 24.5 mpg. Are you suggesting that the computer is more accurate than nearly 300,000 miles of documented fuel logs?
fair enough again, but what do her hand calcs say?
unless the trucks are used identically, it would be hard to assume that one truck has anything to do with another, though.
As you said also, Diesels are different than gas. i know almost nothing about diesel injection, but i don't think they have any means by which to detect or correct for injectors that are not functioning as designed. So, as you said, when hand calcs and the display start differing wildly, one may suspect a fuel injection problem.
I was speaking about gasoline engines, i don't know enough about diesels to speak with any sense of confidence.
for a diesel, i could be convinced.
So far I have purchased 19,194.82 gallons of fuel for my truck. At the time of that purchase the odometer read 298,765 miles. I hand calculate the lifetime fuel economy to be in the 15.5 mpg range ( includes any towing ).
fair enough, what does your readout display? While i applaud your monumental display of attention to detail, to me an overall lifetime average fuel mileage is not very useful. particularly if you are going to assume that anything that deviates from that overall average is wrong.
i'm more interested in trip-trip or tank-tank averages. as in, what did i get pulling my boat to the lake, or going to the hills with the camper. that type of thing. which is prone to some error. using hand calcs to indentify
My wife has an identical truck except it is a Lariat version. The overhead computer says 24.5 mpg. Are you suggesting that the computer is more accurate than nearly 300,000 miles of documented fuel logs?
fair enough again, but what do her hand calcs say?
unless the trucks are used identically, it would be hard to assume that one truck has anything to do with another, though.
As you said also, Diesels are different than gas. i know almost nothing about diesel injection, but i don't think they have any means by which to detect or correct for injectors that are not functioning as designed. So, as you said, when hand calcs and the display start differing wildly, one may suspect a fuel injection problem.
I was speaking about gasoline engines, i don't know enough about diesels to speak with any sense of confidence.
for a diesel, i could be convinced.
I agree that for short periods of time, the computer display will show the trend of milage increasing or decreasing depending on loading, towing, heavy foot of the throttle, etc. It has better knowledge of the current fuel consumption. Also, over a short period of time, one can assume that the injectors continue to deliver the same amount of fuel per millisecond, or whatever time unit is used.
agreed. but keep in mind that those instantaneous readings are what is used to calculate the average. if one is to be believed, the other should be as well.
Without knowledge of EXACTLY how much fuel the injector delivers (calibrating), the mpg displayed can deviate from the true mpg, but the offset should be consistent. This is because the computer believes the injector is delivering a particular amount of fuel, but has NO WAY of knowing how much fuel is ACTUALLY delivered. This is because neither the volumetic flow rate of the fuel, nor the mass flow rate of the fuel is ever measured. It has been established that air flow is measured, DESIRED fuel calculated, and injector opening time commanded.
the PCM detects any difference between the fuel commanded and the fuel delivered via oxygen sensor. then uses the trim tables to update the look up (flow rate) tables. this is how the PCM can correct for changes in fuel injector performance over the long term.
When the desired amount of fuel delivered and the actual amount of fuel delivered differ, how does the computer know? It doesn't. Until the difference becomes so great it impacts drivability or triggers an emission control problem.
again, the oxygen sensor detects this difference. an emissions problem will only show up to the PCM once the flow rates get out far enough that the trim can no longer compensate.
again, for gasoline only. for diesel, you are likely correct.
I expect most folks would agree that tank to tank milage estimates are inconsistent, but multi-tank milage estimates are much better. Over multiple fill-ups the difference in where the nozzle cuts off the fuel becomes less significant.
agreed
The example Shake-N-Bake cited for lifetime milage is kind of an extreme example - the difference is where the nozzle cuts off the fuel is nothing compared to over 19 thousand gallons!
agreed
I use the computer as a guide for milage getting better or milage getting worse. I use my hand calcs for what my actual average milage is.
so how does your many tank average compare to the computer averaged over the same period of time? i've never really tried it, but if you are correct, the two numbers should start to converge as the errors in tank fill start to become increasingly insignificant.
agreed. but keep in mind that those instantaneous readings are what is used to calculate the average. if one is to be believed, the other should be as well.
Without knowledge of EXACTLY how much fuel the injector delivers (calibrating), the mpg displayed can deviate from the true mpg, but the offset should be consistent. This is because the computer believes the injector is delivering a particular amount of fuel, but has NO WAY of knowing how much fuel is ACTUALLY delivered. This is because neither the volumetic flow rate of the fuel, nor the mass flow rate of the fuel is ever measured. It has been established that air flow is measured, DESIRED fuel calculated, and injector opening time commanded.
the PCM detects any difference between the fuel commanded and the fuel delivered via oxygen sensor. then uses the trim tables to update the look up (flow rate) tables. this is how the PCM can correct for changes in fuel injector performance over the long term.
When the desired amount of fuel delivered and the actual amount of fuel delivered differ, how does the computer know? It doesn't. Until the difference becomes so great it impacts drivability or triggers an emission control problem.
again, the oxygen sensor detects this difference. an emissions problem will only show up to the PCM once the flow rates get out far enough that the trim can no longer compensate.
again, for gasoline only. for diesel, you are likely correct.
I expect most folks would agree that tank to tank milage estimates are inconsistent, but multi-tank milage estimates are much better. Over multiple fill-ups the difference in where the nozzle cuts off the fuel becomes less significant.
agreed
The example Shake-N-Bake cited for lifetime milage is kind of an extreme example - the difference is where the nozzle cuts off the fuel is nothing compared to over 19 thousand gallons!
agreed
I use the computer as a guide for milage getting better or milage getting worse. I use my hand calcs for what my actual average milage is.
so how does your many tank average compare to the computer averaged over the same period of time? i've never really tried it, but if you are correct, the two numbers should start to converge as the errors in tank fill start to become increasingly insignificant.
Forget 5 gallons. Fill the tank to the same point by removing the flapper in the fill tube, scribing a mark you can see near the top of it, and fill it to that point. That gets the repeatable fill you are seeking.
That would work too, but 5 gallons is the calibrated amount, so it would be the most accurate, that's why i say to use exactly 5.000 gallons.
That would work too, but 5 gallons is the calibrated amount, so it would be the most accurate, that's why i say to use exactly 5.000 gallons.
words added to fool browser[/COLOR]
Super Moderator
Joined: Aug 2003
Posts: 25,479
Likes: 742
From: Isanti, MN
This is a cool thread. Here's my seal of approval:
On a serious note, while I agree that the PCM adjusts fuel trims based on the oxygen sensors, how does it know exactly how much fuel is being burned? Is the MAF sensor 100% accurate?
Something to think about. I'd like to add that I have had a few lie-o-meters ove r the years, and while some of them are consistent I have never seen one consistently right. My Fusion's LOM is almost always 1.5-3 MPGs low based on hand calculating. This is based on always resetting the LOM before starting the car after a fill-up as well as tracking every single tank the car sees.
Fusion (Ford Fusion) | Fuelly
On a serious note, while I agree that the PCM adjusts fuel trims based on the oxygen sensors, how does it know exactly how much fuel is being burned? Is the MAF sensor 100% accurate?
Something to think about. I'd like to add that I have had a few lie-o-meters ove r the years, and while some of them are consistent I have never seen one consistently right. My Fusion's LOM is almost always 1.5-3 MPGs low based on hand calculating. This is based on always resetting the LOM before starting the car after a fill-up as well as tracking every single tank the car sees.
Fusion (Ford Fusion) | Fuelly
Cargo Master
Joined: Oct 2012
Posts: 2,333
Likes: 35
From: South Carolina
Originally Posted by meborder
i'm more interested in trip-trip or tank-tank averages. as in, what did i get pulling my boat to the lake, or going to the hills with the camper. that type of thing. which is prone to some error. using hand calcs to indentify
Super Moderator
Joined: Aug 2003
Posts: 25,479
Likes: 742
From: Isanti, MN
Wanted to correct my previous post, I was tired last night and forgot to proofread.
How does the PCM know exactly how much air is being consumed? Does anyone really think that MAF sensors are 100% accurate? What about oxygen sensors, are they 100% accurate?
Furthermore does the PCM assume fuel consumption is directly related to air intake through the MAF? This would be the only way the PCM could compensate for worn injectors with the help of the oxygen sensors, because under this condition fuel delivered would not be constant with a given injector pulse width. It would vary, which would be why fuel trims exist.
So the only way for the PCM to truly adjust consumption estimates would be for it to ignore injector pulse width and calculate based on the MAF sensor and possibly a correction factor with the fuel trims. But we all know that's not the way it works; in fact the LOM on the older Super Duty trucks is directly wired to read injector duty cycle. Which means that it's not possible for the LOM to calculate based on fuel trims or air consumption, as the LOM doesn't have access to this information. So in plain english, my LOM doesn't care if the injector tips are worn.
Something else to consider is that fuel flow for a given injector pulse width will vary based on fuel pressure at the rail. More fuel pressure means that more fuel would be going through the open injector nozzle in a given time. But here's the catch: all the older SD trucks don't even monitor fuel pressure! The PCM calibrations rely on that pressure being within a given range to deliver the appropriate amount, which is yet another reason for oxygen sensors to be there for mixture adjustment.
So in a nutshell the fact that we have oxygen sensors and variable fuel trims precludes the PCM from being exact on its' fuel consumption. Because if fuel delivery were precise all the time there would be no need for adjustment. And without 100% accurate oxygen sensors, MAF sensors, and MAP sensors there is no way for the PCM to know exactly what's going on without some sort of variance.
By the way my Excursion is also consistently off by ~0.5-1 MPGs.
How does the PCM know exactly how much air is being consumed? Does anyone really think that MAF sensors are 100% accurate? What about oxygen sensors, are they 100% accurate?
Furthermore does the PCM assume fuel consumption is directly related to air intake through the MAF? This would be the only way the PCM could compensate for worn injectors with the help of the oxygen sensors, because under this condition fuel delivered would not be constant with a given injector pulse width. It would vary, which would be why fuel trims exist.
So the only way for the PCM to truly adjust consumption estimates would be for it to ignore injector pulse width and calculate based on the MAF sensor and possibly a correction factor with the fuel trims. But we all know that's not the way it works; in fact the LOM on the older Super Duty trucks is directly wired to read injector duty cycle. Which means that it's not possible for the LOM to calculate based on fuel trims or air consumption, as the LOM doesn't have access to this information. So in plain english, my LOM doesn't care if the injector tips are worn.
Something else to consider is that fuel flow for a given injector pulse width will vary based on fuel pressure at the rail. More fuel pressure means that more fuel would be going through the open injector nozzle in a given time. But here's the catch: all the older SD trucks don't even monitor fuel pressure! The PCM calibrations rely on that pressure being within a given range to deliver the appropriate amount, which is yet another reason for oxygen sensors to be there for mixture adjustment.
So in a nutshell the fact that we have oxygen sensors and variable fuel trims precludes the PCM from being exact on its' fuel consumption. Because if fuel delivery were precise all the time there would be no need for adjustment. And without 100% accurate oxygen sensors, MAF sensors, and MAP sensors there is no way for the PCM to know exactly what's going on without some sort of variance.
By the way my Excursion is also consistently off by ~0.5-1 MPGs.
Senior User
Joined: May 2004
Posts: 407
Likes: 0
From: Humble, Texas
The DTE calculation (Distance To Empty) in my '11 is not as dynamic as I'd like, but it is calculated at when you start the truck. It takes the approximate amount of fuel in the tank, looks at whatever the average MPG is at the moment, and gives you a number of how long it'll take you to run out at that mileage, meaning it has to assume that same average will continue to be accurate for the duration of your trip.
Every once in a while after refueling, the DTE will still say "20 miles to empty" or whatever it said before I shut it down to fuel. Then a couple minutes later, it'll switch to "550 miles to empty" or somewhere thereabouts. So if it was calculated with the conditions it knows at startup, how would it correct itself when it figured out that it had 36-ish gallons more than it thought it had? Or even more interestingly, what if you never shut down to fuel? The display would be wildly off. That isn't the case either.
Secondly, if I were to drive my truck regularly for 2 months, it would have a pretty good idea of how much fuel it is using. Then one day, I go hitch up my 37' fifth wheel camper. After it's all hitched up and I start the truck, you're saying that the fuel consumption numbers are loaded into the DTE calculations. The problem is that if I drive all day long with the camper, the truck should be calculating DTE based on empty numbers. But it doesn't. It keeps up very nicely with how much fuel is actually being used.
My owners manual says: "DTE is calculated using a running average fuel economy, which is based on your recent driving history of 500 miles." Through experience, I know that the DTE display is constantly learning when fuel use drastically changes, such as when a trailer is first hitched up or it is dropped off. I don't think the conditions at startup are the conditions that it always uses. I have a hard time accepting that a model year 11 would go backwards in this regard from a model year 05.
Just my thoughts.
Lead Driver
Joined: Oct 2010
Posts: 5,420
Likes: 16
From: Ouray, CO
I observe similar things.
i feel the dte is using the last, say 50 mi.
The sg2 gives inst avg along with trip data.
On flat highways it seems the values reflect better.
City driving or towing the averages swing so much the
Values are less likely to be close.
sort of like dead reckoning when you change course/speed
a lot with a strong wind on a hill.
i feel the dte is using the last, say 50 mi.
The sg2 gives inst avg along with trip data.
On flat highways it seems the values reflect better.
City driving or towing the averages swing so much the
Values are less likely to be close.
sort of like dead reckoning when you change course/speed
a lot with a strong wind on a hill.
Thread Starter
|
Posting Guru
Joined: Sep 2009
Posts: 2,015
Likes: 4
From: Lakehills, TX
Unfortunately, it does appear to be that way. In the info center you can select between towing and normal DTE, though I'm not sure how it gets that info, unless it's going purely off of use of the tow/haul mode utilization. It does not appear to automatically select that mode based off of a recognized trailer being hooked up to it (I have two named trailers it tracks, the 20-ton gooseneck and the 5er). In the end, I wish it updated itself far more frequently to account for differing usage, but it is what it is...
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
Top 10 Most Expensive Ford Trucks Ever Sold on Bring a Trailer
Joe Kucinski
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Top 10 Ford Truck Tragedies
Joe Kucinski
AEV FXL Super Duty - the Super Duty Raptor Ford Doesn't Make
Brett Foote
Lobo Vs Lobo: Proof the F-150 Lobo Should Be Even Lower!
Michael S. Palmer
Ford's 2001 Explorer Sportsman Concept Looks For a New Home
Verdad Gallardo
10 Best Ford Truck Engines We Miss the Most!
Joe Kucinski
2026 Shelby F-150 Off-Road: Better Than a Raptor R?
Brett Foote
2027 Super Duty Carhartt Package First Look: 12 Things You NEED to Know!
Michael S. Palmer
Cargo Master
Joined: Sep 2007
Posts: 2,624
Likes: 7
From: OR and UT
Now you are living up to your user name. 
I tried several times to sit down and read through every post in this thread, but without my pocket protector it just ain't clickin'.
I tried several times to sit down and read through every post in this thread, but without my pocket protector it just ain't clickin'.
Posting Guru
Joined: Aug 2007
Posts: 1,858
Likes: 6
From: laporte in
This has been a very informative thread. Lots of different info!
I fill my truck up once a week normally. My lie-o-meter has been mostly dead on with my hand calcs. A few times I could say it most was .2 off. And that was weeks when trailers were pulled a lot.
I fill my truck up once a week normally. My lie-o-meter has been mostly dead on with my hand calcs. A few times I could say it most was .2 off. And that was weeks when trailers were pulled a lot.
Senior User
Joined: Mar 2004
Posts: 352
Likes: 0
From: Moose Jaw Saskatchewan
Just came back from a 990 km round trip from Moose Jaw to Minot and the computer said we used 158.4 liters, we actually used 162.1 liters so it is pretty darn close. thats .95 of a gallon out in 618.5 miles.
Thread Starter
|
Posting Guru
Joined: Sep 2009
Posts: 2,015
Likes: 4
From: Lakehills, TX
What you mean is that you managed to fit 162.1 liters into the tank, all said and done?
Senior User
Joined: Mar 2004
Posts: 352
Likes: 0
From: Moose Jaw Saskatchewan
My 2011 on the trip meter shows hours running, liters used, trip average liters per 100km and total km driven for the trip so it is an easy caculation. And I actually pumped 162.5 liters into it, at least that is what I was pretty sure I said.