MPG Computer Accuracy

[HawkX66]

Shutterbug57, I drive a '15 limited with about 42k miles and ever since it was new the computer mpg has consistently been approx 1.4 to 1.6 mpg higher than when I hand calculate it.
I too don't understand why this is, or how they calculate mpg. Must use some kind of "new" math other than how many miles traveled divided by how many gallons it took to drive those miles.

For 3,000 miles mine has shown exactly the same. It doesn't matter if I drive highway from fill up or it's stop and go. The end result is close to the same variance.
Keeping track of mpg is a good indicator of how well your truck is running or whether something needs attention.

 

[TobyU]

How do you calculate it? Different people do it different ways and then many people claim they do it the most accurate way but I'm not quite certain about that. There's only one perfectly accurate way to do it and that wouldn't be good on your fuel pump.

 

[HawkX66]

It's pretty straight forward. Fill up, drive X miles, fill up. Divide miles driven by gallons used. Repeat. Once you've done this a few times you can get a pretty accurate average.

 

[Artie]

How do you calculate it? Different people do it different ways and then many people claim they do it the most accurate way but I'm not quite certain about that. There's only one perfectly accurate way to do it and that wouldn't be good on your fuel pump.

I use the ‘car minder’ and ‘AUTOsist’ apps for iOS to track mine.

 

[TobyU]

It's pretty straight forward. Fill up, drive X miles, fill up. Divide miles driven by gallons used. Repeat. Once you've done this a few times you can get a pretty accurate average.

Yes and the key term is pretty accurate. So if people are saying that their in car mpg reading is 1 mile per gallon higher or something there is still room for error in people's testing procedure when they do it manually.
There's no real way to know that you're feeling it the same every time unless you're crazy like I did a few times on one of my Town Cars and filled it till it was literally coming out of the neck of the car but that takes a long time.
That's a little more accurate though. To be extremely accurate you would have to do like MythBusters does and have external fuel tank for you had a set amount you can measure exactly. Or the heart of the fuel pump weigh as I mentioned. You'd have to run the car until it ran out of fuel. Put exactly 5 gallons in and then drive it again until it ran out of fuel again and you know exactly how far you went on 5 gallons.

 

[HawkX66]

We're not talking about accuracy for the space shuttle. Most of us are just trying to get a "pretty accurate" measurement. If you fill it the same way each time, air space in the tank anomalies etc will become less and less of a factor. That's why you take the average of all the averages ignoring any outliers.

 

[aggiegrad05]

We're not talking about accuracy for the space shuttle. Most of us are just trying to get a "pretty accurate" measurement. If you fill it the same way each time, air space in the tank anomalies etc will become less and less of a factor. That's why you take the average of all the averages ignoring any outliers.

Yep. Yes, unless you fill up at the same actual pump at the same time of day every time there will be anomalies, but if you do it using the "fill up-drive-fill up-divide miles by the number of gallons" method many multiple times and the computer is always high, that will scrub out most of the anomalies and tell you that the computer is high.

Another good equalizer is the lifetime average. The day I bought the vehicle I reset Trip 2 to 0 and have never touched it. So that shows what my truck believes to be it's lifetime average MPG...and it's 1.4 mpg higher than the combined average of all my hand-calculated fill ups.

 

[TobyU]

Yep. Yes, unless you fill up at the same actual pump at the same time of day every time there will be anomalies, but if you do it using the "fill up-drive-fill up-divide miles by the number of gallons" method many multiple times and the computer is always high, that will scrub out most of the anomalies and tell you that the computer is high.

Another good equalizer is the lifetime average. The day I bought the vehicle I reset Trip 2 to 0 and have never touched it. So that shows what my truck believes to be it's lifetime average MPG...and it's 1.4 mpg higher than the combined average of all my hand-calculated fill ups.

I just don't worry too much about it being a mile per gallon higher. Maybe they just do it to make us feel better. I'm okay with that. I like to feel better more than I like accuracy. But don't get around any engineers. They will lose their minds. Then they will take stuff apart especially lawn mowers and damage them and cause more problems for the tech to fix them. Like the two this month that took their Muffler off to fix what they thought was a fuel delivery issue. One even thought it was the carburetor!
I just know the mpg meter is quite accurate. The harder I drive and the more I idle, the lower it goes.

 

[HawkX66]

Maybe they just do it to make us feel better. I'm okay with that. I like to feel better more than I like accuracy.

Wow, just wow....

 

[TobyU]

Wow, just wow....

Why in the world does that wow you?
As long as every Ford product or every car has the numbers slightly higher then we can still all compare them. Since we're in an expedition forum we can assume that all the Expeditions for a relative number of years are all going to be the same amount of an accuracy so we can compare what we get to what others get.

Does it wow you that I could care less if my auto MPG indicator is less than 10% off? Or does it bother you more that they are?
I have just basically always use one as a relative up or down for how my car is running or in current conditions. I've never really cared about the exact number is. When the tank gets low I have to put more gas in whether I was getting 12 miles to the gallon or 25 miles to the gallon.