How exactly does this thing work? When I first got my truck it sat at an average of 'around town' driving at 13.4 Mpg - 13.6 Mpg. I reset it the other day, and it now fluctuates quite a bit. Letting my truck warm up in Park, it dropped 2 Mpg. Going up a hill with foot on the gas it drops a couple of tenths, coasting down a hill it goes up a couple of tenths etc... How long does it take for it average out and find 'it's zone' so to speak? Any input would be appreciated.
--20" Wheels/Stock Pirelli Tires

Thanks,
Steve

 

i wouldnt exactly trust this readout to be acurate. do the old fashioned way for best results

 

the MPG readout is based on the entire tank and an algorithm.

the more fuel in the tank, the quicker you see changes in the readout (like idling...or flooring the motor)

the less fuel, the slower it changes.


overall, the MPG readout is a best guess for your MPG. Mine is only good for being within 1 to 1.5 mpg. pretty much serves the same function as those old fuel economy shift lights

 

If it you reset it a lot it can be somewhat accurate. I've head it within .1 of actual mileage and I've seen it as much as 1 1/2 off. It just depends on how much you reset it. Either way, it's a best guess although many people on this board will tell you it's "dead accurate" which is BS.

If you really want to know write down your mileage everytime you fill up and then divide by how many gallons you put back in it. I keep a piece of paper in my truck and write it down everytime.

 

It compares fuel used to miles driven since the last time you reset it. It's the same math you use when you calculate it manually, but the only way you can compare the two is if you push the rest button every time you fill-up.

When you push the reset button there is no data and it reads 0.0 for a while. Then, when you have driven a mile it calculates the fuel used in that mile. If your coasting down hill the mpg will be high, if your starting from a stop it will be low. At first, after you reset it, the number will change quickly because there is little history data and each mile has a bigger impact on the number. If you never push the reset button then there is a lot of data and the number will be more stable.

If you have never reset MPG, then it shows the average miles per gallon for the total miles on the vehicle.

 

Hate to say you're wrong Tylus, but in this case you are.

The MPG in the Lariat control center is simply based on AVERAGE fuel consumption. Period.

It has ZERO to do with the amount of fuel in the tank. ZERO. When you reset the MPG parameter, it is starting it's averaging all over again; hence, the extreme fluctuations at the beginning.

So, if you reset it, it will seem to go up and down vigorously when the truck starts from a stop (using a lot of fuel), accelerating (using a lot of fuel)..... The flip side is if you reset it and coast down a slight decline, you will see your average MPG hit north of 40mpg. It's an average. Plain-n-simple.

If you want an instantaneous readout, reset it. If you want an average, leave it alone.



bigred... You too are very wrong. The MPG is dead on accurate! I have taken plenty of long (300+ miles) trips and that readout was accurate to the 10th.


Well Jeremy, all I can say is that yours must be defective. Resetting it repeatedly makes no difference in the way it works. Get yours checked out. And for it to be 1 1/2 off means it's either defective or your fillups are not consistent. It's that simple.


Yup, pretty accurate.

 

GreenCrew has it (almost) exactly correct.

The computer calculates the MPG starting from the last reset, but not for the total miles driven since the last reset. The system keeps collecting miles and gallons used info until it reaches (I think) 500 miles. It then continues by using the most recent 500 miles driven, dropping off the old data and adding in the new data.

I have found the MPG number to be pretty accurate over the long term. If you manually check MPG each time you fill up, it will be different from the MPG readout that the truck shows, but if you calculate your MPG using several tankfuls, you will find that the MPG readout will probably be pretty close.

 

Where exactly did you find this data? Where did this magical " 500" mile number evolve? Call me skeptical, but unless this was from a Ford engineer, I highly doubt it's accuracy. Seems like someone's guessing on this .....


This doesn't even make sense? This might be accurate if one fills up their tank every 25 miles or so. The fillup method would not even be accurate for anything less than 50 miles due to inconsistencies from one pump to the other. The MPG readout is accurate ALL THE TIME, regardless of the miles driven. Floor it and the MPG may drop. Coast and it will rise.

 

No need to be so confrontational.

You are correct about the 500 miles. The 500 mile number is actually what is used to figure the running average fuel economy that is then used to calculate the distance to empty number. I did not guess at this, I simply remembered what I read incorrectly.

From the owners manual:

As far as the average MPG displayed on the message center, I have not found anything that says exactly how this number is calculated. I have, however, done what I suggested, as far as manually calculating MPG over several tanks of gas and I stand by my statement. Each tank of gas that I use produces a different MPG number which is a function of how the truck was driven while using that tank of gas. If I add all the gallons purchased over several (5 or so) tanks full of gas and divide that by the number of miles driven using those several tanks of gas, that MPG number will be very close to the MPG number being displayed on the message center. Try it for yourself and see.

 

Well, sir. It's been awhile since I reset at the time of fill-up. I just quit screwing with it after awhile. When I did I don't remember it being 100% dead on accurate like you claim but it was close. If you do not reset it though it's way off from average fill-up.

I drive a ton for work so I'll start monitoring it again. I think it would be more accurate on a long trip than stop/go city that's for sure. There's far more flucuation on city driving.

 

No confrontation intended. Sorry.

When in doubt, check the manual.

BUT, as you correctly stated, that number is derived for the DTE number, NOT the MPG number.

I believe that pump inconstancies will be far more likely to skew the actual MPG than the trucks computer, which in the end is the device metering the fuel that the engine actually uses.

Unless one fills up at the exact same pump at the exact same filling station, how can one be 100% sure that what they are calculating is "exact" ? Fortunately, we're only talking about MPG down to the nearest gallon. If one wants to be more accurate than that (say to the tenth), then I would put more faith in the trucks computer, than guessing what the shut-off was calibrated at and what the flow rate for the particular pump nozzle is......

 

Absolutely.

The onboard computer is using an exact quantity of gasoline used to drive an exact number of miles, whereas the manual method uses the number of gallons used to REPLACE the gallons used to drive those miles. There is no easy way to be sure that you are putting back into the tank the exact same volume of gas used to produce those driven miles.

Plus the fact that the onboard computer is giving an average MPG which is being calculated for more than one tank full of gas, unless, of course you reset the display each time you fill up.

I am still curious to know exactly how the average fuel economy number is calculated. Is it based on every mile driven since the last reset, or is there a maximum number of miles reached at which time the computer dumps the old data and adds new data. I can't seem to find an answer to this question.

 

Some good points. We all have used the manual calculation at the pump which, due to large quantities of gallons and miles, is accurate enough for our needs. I think there are three reasons though why we can question the accuracy of that method to represent MPG on a tank by tank basis. If you reset the MPG on the message at the pump and the number does not exactly match your manual calculation, I believe that is because it is your measured gallons which are not as accurate.

 

Try a Scangauge II, they are very accurate.