Inconsistent difference between displayed and actual mpg

[mwr]

My actual mpg (dividing the tank miles by the gallons to fill) has always been about 2 mpg less than the displayed mpg for that tank. That also seems to be pretty common here. But my last tank displayed 44 mpg and the actual/calculated mpg was 45 That's about what I would expect the actual to be, but with a displayed mpg of 47 instead of 44.

 

I think if the pump used for the last fillup didm't put in as much as the "normal" pump, that would cause that result. But this pump was the one I've used most often.

 

It's really not a problem, but any ideas about it?

Edited July 4, 2015 by mwr

[DeeCee]

My experience has been that the car's displayed mpg is about 1.7 to 2.5 mpg higher than the actual calculated mpg. My current lifetime display is 48.1 while the actual is 45.9 after nearly 45,000 miles.

[mwr]

My experience has been that the car's displayed mpg is about 1.7 to 2.5 mpg higher than the actual calculated mpg. My current lifetime display is 48.1 while the actual is 45.9 after nearly 45,000 miles.

My experience is the same as yours except for this last tank.

[djminfll]

Mine varies pretty substantially - sometimes, I get better actual economy than the displayed mpg, but not always.

[lolder]

Measuring mpg by gallons filled to pump shutoff is not very accurate for one fill up. The car's computations are more precise. The error in mpg in the computer displays is constant. It is about 4% high. The odometer is about 2% low, however. The net error is about 2% high or about 1 mpg. There's no need to always compute the mpg with the gallons filled.

[mwr]

The error in mpg in the computer displays is constant. It is about 4% high. The odometer is about 2% low, however. The net error is about 2% high or about 1 mpg.

Why would there be a constant built-in error for those two items?

[corncobs]

Why would there be a constant built-in error for those two items?

To make you FE look better :)

 

That's very common practice across all manufacturers. I don't think the percentage of car owners actually calculating the MPG from miles driven and gallons tanked is very high.

 

Our Explorer is off by the same percentage but the since the MPGs are much less it seems more accurate.

[milleron]

My actual mpg (dividing the tank miles by the gallons to fill) has always been about 2 mpg less than the displayed mpg for that tank. That also seems to be pretty common here. But my last tank displayed 44 mpg and the actual/calculated mpg was 45 That's about what I would expect the actual to be, but with a displayed mpg of 47 instead of 44.

 

I think if the pump used for the last fillup didm't put in as much as the "normal" pump, that would cause that result. But this pump was the one I've used most often.

 

It's really not a problem, but any ideas about it?

 

It's my experience that the automatic shutoff at the pump can vary from fill to fill even when using the same pump . . . which in this day and age of stations with 8, 10, and 12 pumps, is nearly impossible. I'doubt that it could vary enough to account for a 10% change, but two fuelings in succession, one 5% higher than normal and the next 5% lower, might give you the result you report. I suspect that from now on, you'll go right back to the same difference between displayed mileage and actual mileage that you've always observed. Please update the thread after the next tank.

[Texasota]

My experience has been that the car's displayed mpg is about 1.7 to 2.5 mpg higher than the actual calculated mpg. My current lifetime display is 48.1 while the actual is 45.9 after nearly 45,000 miles.

That has also been my experience. I have tracked the MPG for ~12,000 miles and the trip computer has over reported it by 2.25 MPG compared to actual. But it has varied a lot from tank to tank with 8.06 MPG the worst over reporting and -3.26 the worst under reporting. The trip computer has more variance in this car than any car I have owned. So far, this is the only thing I have found to complain about with this car but it is a very minor annoyance in the overall scheme of things.

[lolder]

The trip computer may vary a lot because of all the things that affect mileage; temperature, wind, beginning and ending state of charge, etc. The various car computed mpgs get their data from the fuel delivery system and that is very precise.

[Texasota]

The trip computer may vary a lot because of all the things that affect mileage; temperature, wind, beginning and ending state of charge, etc. The various car computed mpgs get their data from the fuel delivery system and that is very precise.

When I used the term trip computer I meant the MPG as reported by the Trip1 display. I always reset Trip1 each time I refuel and the MPG reported by the Trip1 display is what I compare to the value computed in my spread sheet (miles traveled from Trip1 divided by gallons to fill the tank).

[hybridbear]

That has also been my experience. I have tracked the MPG for ~12,000 miles and the trip computer has over reported it by 2.25 MPG compared to actual. But it has varied a lot from tank to tank with 8.06 MPG the worst over reporting and -3.26 the worst under reporting. The trip computer has more variance in this car than any car I have owned. So far, this is the only thing I have found to complain about with this car but it is a very minor annoyance in the overall scheme of things.

I would say that the variation is not the trip computer, but rather the variation is from the gas pump. Like lolder said, the computer's margin of error is constant and does not vary from tank to tank.

[Texasota]

I would say that the variation is not the trip computer, but rather the variation is from the gas pump. Like lolder said, the computer's margin of error is constant and does not vary from tank to tank.

I agree 100%. But as I indicated earlier, this varies more from tank to tank than any car I have owned. I have grown accustomed to this variability and it is not a big deal but it is interesting that it varies so much with my FFH.

[Cobra348]

I agree 100%. But as I indicated earlier, this varies more from tank to tank than any car I have owned. I have grown accustomed to this variability and it is not a big deal but it is interesting that it varies so much with my FFH.

I've never trusted the car computer for accuracy but what I know is that my average MPG from receipts so far is 44.1 and lifetime MPG of the car is 45.2 - only 1.1 off. Trip meter MPG varies widely so I discount it. I do have to say that the FFH computer seems to be less deviant than the Fiesta one which is nice.

[hybridbear]

I agree 100%. But as I indicated earlier, this varies more from tank to tank than any car I have owned. I have grown accustomed to this variability and it is not a big deal but it is interesting that it varies so much with my FFH.

Perhaps the variation is not because of the FFH, but rather due to other factors. For example, if the variation is 1 gallon on a 10 gallon fill that is a 10% discrepancy. But if the variation is 1 gallon on a 20 gallon fill then that is a 5% discrepancy.

[Texasota]

Perhaps the variation is not because of the FFH, but rather due to other factors. For example, if the variation is 1 gallon on a 10 gallon fill that is a 10% discrepancy. But if the variation is 1 gallon on a 20 gallon fill then that is a 5% discrepancy.

This makes sense and could be a big part of it.

 

Interestingly, it is my tanks where I have an unusually high MPG (as reported by the trip computer) that the inflation seems to be the worse. For example, last winter in Florida the trip computer reported 56.00 MPG for the tank but that was inflated by 8.06 MPG and in another tank the trip computer reported 56.4 MPG but that was inflated by 6.01 MPG. Maybe this fits in with your above hypothesis?

[Bob Cooke]

OK this is funny to mw. My actual MPG vs the car reported MPG are off by @ least 12% upto 35%. I just got my car back from the dealer

who did the power steering bolts recall. I explained to him my questions about it. His response..... are you ready for it.

The instant MPG readout on my dash is set to 5 min interval and he said thats what the difference is reporting HUH????

Did I just hear that right my instant readout is causing my tankful MPG to be off. WOW I'm such an idiot.

Here is my fill ups and the actual vs computed MPG see for yourselves. My computer is giving me hints it doesn't want to

do things that I need done right now so it be posted as soon as my computer lets me update the post.

Edited July 11, 2015 by Bob Cooke

[Sleddog]

When I filled up today, using the same pump I usually do, My gallons pumped was .4 of gallon less then the trip meter showed I used. That's the highest difference I've had. Usually, I'm +/- .1 to .2 less or more then the gallons used on the trip meter. My last fill up was 2 weeks ago. When I fill up again in 2 weeks I'll see what the difference is just in case some type of maintenance was done to the pump. My mileage for this tank was 56 according to the car, but the hand calculation was 58.3

 

My driving routine was normal, with a couple of side trips I make once in a great while.

Edited July 11, 2015 by Sleddog

[hybridbear]

OK this is funny to mw. My actual MPG vs the car reported MPG are off by @ least 12% upto 35%. I just got my car back from the dealer

who did the power steering bolts recall. I explained to him my questions about it. His response..... are you ready for it.

The instant MPG readout on my dash is set to 5 min interval and he said thats what the difference is reporting HUH????

Did I just hear that right my instant readout is causing my tankful MPG to be off. WOW I'm such an idiot.

Here is my fill ups and the actual vs computed MPG see for yourselves. My computer is giving me hints it doesn't want to

do things that I need done right now so it be posted as soon as my computer lets me update the post.

MPG stats.jpg

Do you use remote start? Any fuel used by remote start is not counted by the trip computer.

[md13ffhguy]

Do you use remote start? Any fuel used by remote start is not counted by the trip computer.

 

I just looked at my numbers recorded in Fuelly, and saw that from January through March (when I used remote start on a semi-regular basis), my discrepancy between dash and reality averaged about 9% over 11 fill-ups, In the 12 fill-ups since then, the discrepancy is only 3%.

[raadsel]

This makes sense and could be a big part of it.

 

Interestingly, it is my tanks where I have an unusually high MPG (as reported by the trip computer) that the inflation seems to be the worse. For example, last winter in Florida the trip computer reported 56.00 MPG for the tank but that was inflated by 8.06 MPG and in another tank the trip computer reported 56.4 MPG but that was inflated by 6.01 MPG. Maybe this fits in with your above hypothesis?

 

Another difference that most people forget is, because you are getting higher mpg, the variations seem larger. MPG does not scale well, at high mpgs a small percentage looks like a big difference. Remember that an 8 mpg difference at 56 mpg is about 14% off; the equivalent on a car that is getting 20 mpg is only about 3 mpg. I'd submit that you really aren't seeing that much difference in variation, compared to other cars you owned, but rather that the difference appears larger.

[Darkside]

I have a suggestion. I'll even give it a try the next few fill ups. So we are saying the pumps are off causing this? Here is my suggestion. What if you put in the gallons it is reporting used and try to get it as close as possible and follow that pattern for awhile? Comments?

[Texasota]

I have a suggestion. I'll even give it a try the next few fill ups. So we are saying the pumps are off causing this? Here is my suggestion. What if you put in the gallons it is reporting used and try to get it as close as possible and follow that pattern for awhile? Comments?

Beginning with the 2015s there is no longer any data reported on gallons used. Those metrics have been removed from all of the information displays. That was probably done (speculation on my part) because of inconsistencies that we have been discussing and complaints from owners.

[md13ffhguy]

So we are saying the pumps are off causing this?

I don't think anyone is suggesting wild variability in pump readings, although you never know... The variation at the pump is our inability to know if we've filled the tank to the same exact level/capacity each time. For example, not all pumps are on a level grade, at least not where I live. Sloped one way, the car may be able to hold more fuel than another. Think of a bucket of water that's half full. Now, set that bucket down on a hill. Also, there's the variability that's caused by the calibration of the auto-shutoff feature from one pump to another...

[Bob Cooke]

I don't use remote start in the summer months and usually fill at the same station and pump at about the same time of day.

Not that it's a problem but it just seems to pay so much for a new car with all the technology that it would be closer then 12 - 35% off.

Really other then that the car performs fine. VERY HAPPY with my fuel milage in town to and from work.