C-max MPG not calculated correctly

[mbedit]

I noticed that my MPG was off by about 1/2 a gal (actual) when I purchased my c-max. But since I had the efficiency upgrade done to it last August, its now off by about 1 gal actual.

 

What I mean by this is the reported gallons used for a tank of gas by the trip meter (resetting it when I fuel up) may say 11 Gal, but I have to pump 12 gal to fill it up, thus lending to difference between the actual and reported.

 

I'm pretty sure others have seen this, and I'm wondering if anyone has reported it to Ford, or if there is a TSB that would get it closer to actual?

 

While its nice that my C-Max says its getting an average of 46-47 MPG, I know the actual is closer to 42-43 so its over reporting by almost 4 MPG.

 

[salman]

Mbedit

 

Have you eliminated the possibility of gas pumps shutting off at different levels of "full"? I don't know how much capacity is supposed to be left in the tank when a pump hits auto-shutoff. That might account for some of the discrepancy. But a 1 gallon difference sounds like a lot.

[mbedit]

For experimentation purposes, I have attempted to use the same gas station and the same pump. that aside, all things being equal, if it were the point of pump shut off, one would think sometimes it would over report and other times it would under report.

 

I have considered that if the seal of the "capless" tank were bad that maybe I'm loosing gas due to evaporation? But 1/2-1 Gal of gas evaporating seems like allot too!

 

It might seem picky, but when I owned a 350z, that thing was always dead nuts on about the MPG and gas usage. I find it odd that c-max can't report it correctly!

Edited November 10, 2013 by mbedit

[salman]

For experimentation purposes, I have attempted to use the same gas station and the same pump. that aside, all things being equal, if it were the point of pump shut off, one would think sometimes it would over report and other times it would under report.

 

that's a key point about the lack of under/over variation. I'm persuaded.

[jmckinley]

Same here.  MPG the car reports got better after the upgrade, but the calculated mileage tank to tank didn't change at all.

 

My dark side thinks that Lord Motor Company may have scammed us just a bit to make us think we're getting better mileage.

[mbedit]

Yep. I would say I might have seen a modest (1-2 MPG) increase since the upgrade, however, It reports 46-47 MPG now when its really only getting 43-44 MPG, which is off by 3-4 MPG per tank!

 

I'll probably call the dealer and ask if they can find any TSB, and then maybe call the Ford customer Service and find out if they have a solution. I'm sure this will be the 1st time they've heard of an issue ;-)

[mrobinso8]

Jmckinley is probably correct.  On trips I have set the trip odometer at 0 and mileage at 0 and kept of log of gas purchases.  At end of trip filled up a final time and divided total gallons used into total miles driven.  Compared to what the car said mpg calculated was to the cars computer which was not reset the whole trip.  The car is over optimistic to Ford's benefit by 2.2 to 2.9 mpg.  This is true regardless of miles drive which indicates that the computer in the car add 2 to the computed mpg prior to display.  additionally, this was also true on my 2011 Fusion gas.  Irritated me since my wife's Honda Accord's trip computer was accurate.

Edited November 12, 2013 by mrobinso8

[Edsel]

This change was probably the biggest part of the update. :drop:

[LizM]

Could this also have to do with rounding?...I notice when I go on short trips, 5 miles or less, perfect weather, using mostly EV, I can show e.g. 63 mpg and 0 gallons used.  Of course I had to use "some" gas in that trip, but perhaps when calculating mpg it only takes the digits out so far.

Edited November 12, 2013 by LizM

[grggwlkr]

I have NEVER filled with equal or less gas than was reported used by the trip 1 display. I always stop filling at the first automatic cut off. I reset my lifetime when I started using Fuelly. The AVG MPG displayed is inaccurate. My lifetime  is 47.8 mpg. Look below for Fuelly lifetime. The only time this is really annoying is when you think you have broken 50 mpg for a tank to find out at the pump, NOT!

[Jus-A-CMax]

Could this also have to do with rounding?...I notice when I go on short trips, 5 miles or less, perfect weather, using mostly EV, I can show e.g. 63 mpg and 0 gallons used.  Of course I had to use "some" gas in that trip, but perhaps when calculating mpg it only takes the digits out so far.

 

Not rounding, truncating. I wish Ford has been consistent with the use of 2 decimal points on every gas used reading, but oh noes....

[bemyax]

While not keeping a written record of the discrepancy, I've noticed the gallons used in Trip 1 is at least a half gallon less than the pump. For consistency, I've used the same pump this whole century. On my most recent fill, the pump measured three fourths of a gallon more than the vehicle report. (!) 

[JAZ]

The typical error for me in over 12,000 miles is between 4% to 8%, much as others are reporting.

[DaveofDurham]

At my 43rd (last) fill-up, my odometer was at 18,445 miles.  According to my car, my lifetime mpg is 43.5 which equates to 424.0 gallons of gas used.  According to my fuelly records (which have the gallons added according to the pump), my lifetime mpg is 41.4 which equates to 445.5 gallons of gas used.  So the running gas used total according to the car is 4.7 percent less than the running sum of the individual pump values.  This is an average of about 0.5 gallons on a 10 gallon fill-up.

 

Of course this only addresses variation in fuel usage.  Months ago there were some posts about the difference between the miles traveled per the odometer and per independent gps.  For example, the thread titled "It Looks Like everyone is getting 1.5% Better MPG's than they think they are!" which indicated that the gps showed about 1.5% more miles traveled than the odometer.  Has anyone looked at this more recently?

[mbedit]

Ok, nice to know its not just me.

 

To me it seems like a pretty simple process. From what i've read the fuel pump is pretty accurate, so the computer knows exactly how much fuel has been consumed and it should know the mileage.... though I guess its under reporting that by 1.5%??

 

But that 1.5% might adjust your MPG by about .5 MPG is all. That would more ignorable than 3-4 MPG I'm seeing.

[DaveofDurham]

I noticed that my MPG was off by about 1/2 a gal (actual) when I purchased my c-max. But since I had the efficiency upgrade done to it last August, its now off by about 1 gal actual.

 

What I mean by this is the reported gallons used for a tank of gas by the trip meter (resetting it when I fuel up) may say 11 Gal, but I have to pump 12 gal to fill it up, thus lending to difference between the actual and reported.

 

I'm pretty sure others have seen this, and I'm wondering if anyone has reported it to Ford, or if there is a TSB that would get it closer to actual?

 

While its nice that my C-Max says its getting an average of 46-47 MPG, I know the actual is closer to 42-43 so its over reporting by almost 4 MPG.

 

 

Your difference between trip meter gallons used and gallons pumped is close to twice what mine is.

 

One thing to check after you fill up is whether you see any signs of gas dripping onto the ground in the vicinity of the filler neck.  I had this happen 5 years ago on my Honda Odyssey.  There was a leak in the filler neck - so right after filling up we would be dripping gas until the level got below the point of the leak.  The Odyssey was 12 years old when this happened.  This is less likely in a new vehicle, but stranger things have been reported in this forum.

[mbedit]

Good idea Dave. I'll do that.

[ptjones]

Ok, nice to know its not just me.

 

To me it seems like a pretty simple process. From what i've read the fuel pump is pretty accurate, so the computer knows exactly how much fuel has been consumed and it should know the mileage.... though I guess its under reporting that by 1.5%??

 

But that 1.5% might adjust your MPG by about .5 MPG is all. That would more ignorable than 3-4 MPG I'm seeing.

EPA testing is done with Pure Gas for one and All the cars and trucks I have owned showed better MPG's than what they actually got. It does look better for them. :)  I do think that the BTU's very with Gas Suppliers. Sometimes I'm pretty close and others off by a gal.

 

Paul

Edited November 14, 2013 by ptjones

[mbedit]

Interesting ptjones. I guess I don't buy that many cars. My last car was a Nissan 350z, and it was always dead nuts on. Most vehicles I owned prior to that one didn't have electronic trip computers, so the 350z is my only real data point. It seems to me that they should be pretty close. Maybe I'm expecting too much?

[salsaguy]

My 2012 Passat also over reports my mpg on its dash by about 1-2 mpg. frustrating for sure.

it may have something to do with the car calculating by the time the car is on even if you don't have the engine on. if you are using the radio in accessory mode it will count that as trip time.

 

EPA testing is done with Pure Gas for one and All the cars and trucks I have owned showed better MPG's than what they actually got. It does look better for them. :)  I do think that the BTU's very with Gas Suppliers. Sometimes I'm pretty close and others off by a gal.

 

Paul

[mbedit]

See this is where I'm at a loss. You can talk all you want about how testing is done with this type of gas or that the clock is running when the engine isn't, but the basic premise of calculating MPG isn't all that complex. I mean its literally the number miles driven dived by the amount of fuel expended. 

 

So if the number is wrong, then either the car is miscalculating the miles, or its miscalculating the amount of gas expended.

 

Its seems from other threads the Millage has been shown to be pretty close via GPS (with-in 1 or 2%), so that means its likely that the fuel expended is wrong... putting aside some other simple issue like a crack in the filler neck.

 

From what people say, the sensor on the fuel pump is pretty accurate. Like I said, most of my cars in the past have been pretty basic, I guess I'm just astonished that something so darn technical and complex as this hybrid car can't get something that seems pretty simple, like accurate MPG to work correctly?!

[mrobinso8]

mbedit - You think Ford wants accuracy?  I think they want the car display to show 2 mpg better to make the car look better.  Lots of people do not comprehend miles divided by gallons used, especially if the calculation is done after several tanks full.

[DaveofDurham]

See this is where I'm at a loss. You can talk all you want about how testing is done with this type of gas or that the clock is running when the engine isn't, but the basic premise of calculating MPG isn't all that complex. I mean its literally the number miles driven dived by the amount of fuel expended. 

 

So if the number is wrong, then either the car is miscalculating the miles, or its miscalculating the amount of gas expended.

 

Its seems from other threads the Millage has been shown to be pretty close via GPS (with-in 1 or 2%), so that means its likely that the fuel expended is wrong... putting aside some other simple issue like a crack in the filler neck.

 

From what people say, the sensor on the fuel pump is pretty accurate. Like I said, most of my cars in the past have been pretty basic, I guess I'm just astonished that something so darn technical and complex as this hybrid car can't get something that seems pretty simple, like accurate MPG to work correctly?!

 

I totally agree that the information used by the car to calculate miles per gallon is the car's measurement/calculation of miles driven and the car's measurement calculation of gallons of fuel used.

 

Several posters have stated in other threads that they found that the car's odometer read 1.5% less miles traveled than did their independent (i.e., not the car's) GPS.  Assuming that the independent GPS units are correct, this 1.5% error in the car's odometer would depress the car's calculated/estimated mpg figures by 1.5%.

 

In regard to gallons of fuel used, several posters have stated in this and other threads that they add more gas when they refuel than the trip odometer reports they have used since their previous fill-up.  I have looked at my lifetime history over 11 months and 18,000+ miles.  The gas the car says I have used (odometer reading at last fill-up divided by lifetime mpg at last fill-up) is 4.7% less than the gas I have put in (sum of individual pump numbers).

 

What I do not understand and have not been able to figure out by searching around the web, is how does the car measure/estimate fuel use.  From my web searching, it appears that the car may start by counting pulses of the fuel injectors and then combine this with a fuel pressure setting or measurement and perhaps other data or constants/factors to come up with its very short term (almost instantaneous) measure/estimate of fuel use and then keep a running sum of these individual values to come up with its measure/estimate of longer term (e.g., from fill-up to fill-up, from trip odometer reset to trip odometer reset, or lifetime) fuel use.

 

Can someone who understands the inner workings of the car explain how the car measures/estimates fuel use and what factors are causing the long term spread we see between the car's info (lower) and what we add at the pump (higher)? 

[jdbob]

From the OBD manual:

 

"Fuel consumed" is continuously calculated based on PCM fuel pulsewidth summation as a percent of fuel tank capacity.

 

I don't see any sign of a flow meter in any of the discussions so it looks like they calculate it based on how long the injectors are on. Probably not the most accurate way to do it.

[spyburn]

I do not know or understand how all this is calculated - I tend to do very simple math.....however, I recently started using my remote start to warm up the car when the morning temps were down in the 40s and below.  Took me a few days to figure out what was happening, but it appears that when my car is running in remote start, using gas and warming up, the trip meter doesn't register the time or the fuel used.  So all of the sudden it was cold and my FE was better than in the past, which sent me looking for an explanation.  My normal mpg heading to work is about 47.  During the cold snap when I let the car warm up in remote start mode I was getting 52.

 

Don't know that this helps.  I hope it didn't confuse the issue.