Fuel economy discrepancy

[greencmax]

Hello, I am new to this forum & recently picked up a new 2013 CMAX hybrid in December last year.

Wondering if anyone else has noticed this issue that I am having & shed some light on.

 

I have ~ 1100 miles on the car now &  always fill up full tank of gas & reset my trip odometer & average FE to zero after every fill up.

I have filled up the fuel tank 4 times in the past 6 weeks. With the temperatures mostly below 32F in SE Michigan area since the past couple of weeks the dash on my CMAX shows Avg FE ~38mpg between fill ups which I thought was ok given the temperatures . But, when I do the math in the conventional way (taking the difference of Odometer readings & dividing the difference by the gallons used) the FE number comes out about 4mpg lower than what is shown on the dash which I think is huge. I drive ~ 60 miles a day to & fro to work with  a mix of city & highway & don't waste gas by warming up the car in the mornings.

 

Please let me know if anyone else has noticed this issue. Could it be something to do with  break-in miles or cold weather ? The discrepancy is too much to swallow. 

[fotomoto]

Welcome to the forum!  :love_shower:

 

Yes the discrepancy is a known issue but yours sounds little on the high side.  I would try to get gas at the same pump a few times to get a better grasp on the situation.  Folks with GPS's report the odo records lower than the actual miles so the discrepancy isn't as much as you'd think. 

 

Differences aside, it sounds like you are doing well for the nasty conditions and probably will be netting 50+mpg in the warmer months; especially after the engine (ICE) gets broken in.   It's the overall yearly avg. that truly matters. 

Edited January 21, 2014 by fotomoto

[cmax-nynj]

 I drive ~ 60 miles a day to & fro to work with  a mix of city & highway & don't waste gas by warming up the car in the mornings.

 

----------

 

I drive 120 miles a day to & from work (not everyday) and I noticed it's better to let the engine warmed up before getting up to the hwy.  When the ICE kicks in and your engine is cold you use more gas, and without a grille cover(despite having active grille shutter) the engine works harder to maintain the optimal operating temp.  Your engine runs cold most of the time as a result.

[ScubaDadMiami]

There is some difference between crunching the numbers and what the C-MAX shows, but that sounds like a big difference, bigger than what I would have figured.  As Doug/Fotomoto mentioned, it could be the gas pump.  I try to fill at Costco, and that has always been very close to what the dash meter shows.

[greencmax]

Thank you for your responses, I use Sams club often for filling up gas &  continue to  monitor the MPG.

I did find someone else had the same issue & had started a thread earlier. 

 

http://fordcmaxhybridforum.com/topic/2973-fuel-guage-mpg-vs-real-world-mpg/

[FordService]

Thank you for your responses, I use Sams club often for filling up gas &  continue to  monitor the MPG.

I did find someone else had the same issue & had started a thread earlier. 

 

http://fordcmaxhybridforum.com/topic/2973-fuel-guage-mpg-vs-real-world-mpg/

 

greencmax,

 

Have you been by your dealer to see what they think? It is definitely a good first step to getting this figured out for you. Once you've got an appointment, send me a PM with your name, phone number, VIN, mileage, and dealer info, and I'll see how I can help. :)

 

Ashley

[hybridbear]

Hello, I am new to this forum & recently picked up a new 2013 CMAX hybrid in December last year.

Wondering if anyone else has noticed this issue that I am having & shed some light on.

 

I have ~ 1100 miles on the car now &  always fill up full tank of gas & reset my trip odometer & average FE to zero after every fill up.

I have filled up the fuel tank 4 times in the past 6 weeks. With the temperatures mostly below 32F in SE Michigan area since the past couple of weeks the dash on my CMAX shows Avg FE ~38mpg between fill ups which I thought was ok given the temperatures . But, when I do the math in the conventional way (taking the difference of Odometer readings & dividing the difference by the gallons used) the FE number comes out about 4mpg lower than what is shown on the dash which I think is huge. I drive ~ 60 miles a day to & fro to work with  a mix of city & highway & don't waste gas by warming up the car in the mornings.

 

Please let me know if anyone else has noticed this issue. Could it be something to do with  break-in miles or cold weather ? The discrepancy is too much to swallow. 

A number of Fusion Hybrid owners (including myself) have found that in the cold weather the dash is less accurate compared to the pump gallons than in the summer. My parents' C-Max Energi is the same way. So is my wife's Prius. Our previous Honda Accord & Saab 9-5 & Saab 9-3 were all that way. It seems to be a pattern.

Edited January 21, 2014 by hybridbear

[ScubaDadMiami]

That seems to add up.  It is very hot here in the summer, and winter is mild and warm, most of the time.  During those times, what I see on the dash indicator and the pump are very close, usually with two-tenths or so of each other.  However, it has been cooler for this tank, and, for the first time, my fill was off by as much as half a gallon.

[roninsd]

I've found that my "actual" (Fuelly.com) mileage is consistently lower than the computer display, usually by a mile or 2.

[Tellio22@gmail.com]

I think there might be an explanation but it would be difficult to determine . I know from working at a gas station when I was a kid that the explanation and contraction rates of gasoline are crazy. I don.t know how all that works when you are pumping full up north in the real Michigan weather but I would suspect that there may be a difference from what you are figuring and what the car automatically figures that is crazy more when the weather is extreme. I"d say go to the dealer as those have suggested however don't be surprised if no solution is determined. Then wait until the weather stabilizes and see if the difference does not fall within a more acceptable margin.

We lost and gained hundreds of galleons when the construction caused the ground temperature to change it was crazy. Also recently I drove to NC from FL and back and the difference on my CMax increased but not as much as you are experiencing however the lowest temp was about 18 F.

I only use the gage as an estimate, my log tells me the real truth I wonder how the computer measures fuel amounts to know how much fuel the car is using. It must be interesting.

[HannahWCU]

I wanted to quantify how much the difference was from what the pump showed and what the CMax showed.  Over my last 7 fill-ups I have averaged a 5.9% lower reading for the CMax vs. pump.  For a car that gets 45 MPG this represents over a 2.6 MPG difference between what the CMax trip computer displays and my actual mileage.  I believe this is unacceptable.  My CMax needs its 30,000 mile oil change next week. I will be talking to the service manager about what can be done. 

[ptjones]

I wanted to quantify how much the difference was from what the pump showed and what the CMax showed.  Over my last 7 fill-ups I have averaged a 5.9% lower reading for the CMax vs. pump.  For a car that gets 45 MPG this represents over a 2.6 MPG difference between what the CMax trip computer displays and my actual mileage.  I believe this is unacceptable.  My CMax needs its 30,000 mile oil change next week. I will be talking to the service manager about what can be done. 

If you check your odometer with GPS you will find that you are going 1.5% miles farther than you think you are and your mpg's are .5-.7mpg better.  5% isn't that bad, I usually see .4-1gal difference with occasionally right on.I don't think the dealer can calibrate the computer, but it is worth a try and lets us know what they say. :) 

 

Paul

[ScubaDadMiami]

Let me throw this out there. I don't know what I am talking about, but I'd like somebody that does to please address something that has been on my mind out this discrepancy thing.

 

I can't help but to think that the meter that is determining actual fuel use must be a pretty accurate piece of equipment, and that it bases its numbers from a source that is very close to where fuel is actually entering the engine. Couldn't the difference in fuel used be accounted for by things like evaporation and changes in temperature?

 

Personally, I have had times where the gauge and the fuel pump are almost identical, but I have also had other times where there is a spread. I almost always fuel up at Costco, which I would imagine is a place that does a good job of maintaining their pumps. So, why would there be times where the two sources pretty much agree along with times where they don't?

 

Before I even know that there was this issue, I started off by going by what the C-MAX gauge said when it came to adding my numbers to Fuelly. I am sticking with what I started. You can subtract a given fudge factor from my Fuelly stats, if you wish. I am sticking with the system that I have used from the beginning, and I am not going to worry about it too much.

[plus 3 golfer]

Actually, the number we should be looking at is $/mile as the money going out of our wallets is real. :)  The gallons we think we filled up with and the gallons the car said we used are not.

 

There is no flow meter that measure fuel used.  It really depends on how Ford is calculating FE using sensor data, injection pressures, injection pulse width, emissions data, tank gauge and so forth, what assumptions they use in the calculation, and  what corrections Ford might make (if any) over time to the calculation.  

 

As far as temperature, the fuel is stored in underground tanks that will have a fairly constant temperature year round.  Yes, if no one has used the pump in hours then the first few gallons out of the pump could be at ambient temperature which could be significantly different than the temperature of the fuel in the storage tank. Also, IIRC some state require fuel pumped to be temperature corrected.   As far as evaporation, modern cars all have EVAP systems so fumes don't escape into the atmosphere.  But, see below as to the purged fuel from the EVAP system may not be counted.

 

Below are recent quotes by GM and Honda on the subject.  Also, note in the link the suggested procedure to calibrate the displayed value.  On my 2009 Jetta I did similar and could adjust a multiplier in +- 1% increments via a scan tool ($250) to get the displayed FE more in line with tank hand calculated.  The problem is that conditions change over time such as tires wear / tire replacement and as such the distance recorded by the odometer changes versus a standard mile.    

 

Mark Allen, GM’s director of global energy, mass, and aerodynamics, says some inaccuracy is unavoidable. “The density of fuel varies. We have no way to measure it. Mobil might be different than Shell. Summer gas to winter gas could be a big difference.” And then there are vapor-recovery systems. With modern cars’ sealed fuel tanks, gasoline vapor accumulated in the tank is absorbed by a charcoal-filled canister. Periodically, this canister is purged by the engine. “If the weather is hot, you generate lots of purge,” says Allen. “This unmetered fuel isn’t counted by the trip computer.”

 

Honda uses additional inputs to calculate trip mpg. “We look at the fuel-consumed data that comes from the engine-control computer, but we also track the float sensor measuring the fuel level in the tank,” says Raj Manakkal, chief engineer for electrical and infotainment devices. He also points out that, due to temperature changes, plastic fuel tanks can expand and contract by as much as a liter. On the Acura ILX, that yields a total variation of 4 percent.

 

Edited March 22, 2014 by Plus 3 Golfer

[Noah Harbinger]

I've been keeping track of the pump's claimed volume since my third tank, and it has worked out to, the pumps claim 3.3% more dispensed than the trip-meter claims was consumed. 

 

[SThompson]

I have 1500 miles on my car, i am not a techy person, or fanatical about gas mileage but 600 miles out of 10 gallons of gas is 60 mpg. Not bad considering the only change in driving habit is slow starts and stops. The display shows 41.7, i guess thats lifetime since i do not reset anything, but actual calculation says 60 and not even broke in yet. I'm very pleased.

[greencmax]

As a follow up to my original post here are some more of my findings now that my CMAX has ~ 4800miles.

 

In summary... there are 3 different mpg numbers I am monitoring.

#1) Average FE -----> This is the grand average FE displayed of all trips, which I've learnt not to re-set.

#2) Trip FE ---> which is FE that I use between fill ups, I reset it after every fill up

#3) Pocket book FE ---> my own calculation of total miles/ gals used what the pump displays, conventional method 

 

# 2 has always come out ~2 to 3 mpg higher than #3.

# 1 is always least (so far, but comes closer to # 3). 

 

I believe #1 paints a realistic picture of the  car's Avg FE.

 

 

I've consistently used Sam's club gas station , usually within 3-4 miles of driving after the low fuel indicator comes on & one time I filled up at a  different location , Exxon, out of curiosity. 

Exxon claims it dispensed 12.5gals when I was expecting the number to be ~ 11.5 gals which is what a typical full tank would take.

And the car tells me it used only 10.7 gals. 

I've come to believe that root cause of the discrepancy lies in the the post that Noah Harbinger posted. Thank you Noah H.

 

Also, 35F ambient temperature seems to be tipping point. As we in SE Michigan come out the deep freeze , the Avg FE is I am getting ~ 41.8mpg & has been consistently moving up. 

Yesterday evening & this morning to & fro to work  the trip summary shows 53 mpg after a 26mile drive one way.

 

Love my CMAX !

[ptjones]

greencmax I wasn't sure what your #3 line meant "(3) Pocket book FE ---> my own calculation of total miles/ gals used what the pump displays, conventional method" Are you using a GPS to keep track of your mileage? I have been using Actual mileage (GPS) from the beginning for my Fuelly mileage, for me it improves MPG's by 1.5%.  My Fuelly average is 45.1mpg and my Lifetime on Smart Gauge is 46.1mpg or about 2% difference. I think 2% difference is no big deal. IMO :)

 

Paul