Cmax computer lies about fuel used vs. actual fuel used measured at the pump

[MADCMAX]

We have had our Cmax hybrid since September 2012, one of the first to come to the dealer.  And we have about 12,000 miles on it at this point.  The nonsense about the break-in period and all that is just nonsense.  Our MPG now is the same as it was when we got it.  

 

We are very conservative drivers and I have been tracking my fill-ups on the EPA website, the fuel economy.gov one, and have an overall average of 37MPG.  I have been tracking it there since November 2012.  

 

When I do my mileage calculations, I measure actual miles in between fill-ups and then actual gas used as shown on the pump.  A huge thing that you probably didn't know unless you do your mileage this way is that the Cmax says you used LESS gas than you actually do.  So you are using MORE gas than the computer thinks and therefore your ACTUAL MPG is less.  The computer might be saying you are getting 42MPG but is actually 37MPG using actual calculations vs. the computer.   

Too many people rely on the computer as being correct - and on many cars they are.  But when you consider 1/2 gallon to 1 gallon MORE GAS USED than the computer says, it can drop your actual MPG enough to make you really frustrated.   I wonder if Ford has planned it this way, to make you THINK you are getting better MPG than you really are.  I take pictures of the gas receipt and the computer trip odometer when I fill up the tank, to show more gas is used than the computer says.   

 

The other thing I have read is that people will see that they took a trip to the store and got XXMPG.  BUT that is not a real number.  Why do I say that?  How many times have you seen 56MPG on the trip summary but used 0 gallons of gas?  Those little parts of a gallon that don't show are probably the reason the car used MORE GAS than the computer says it did.

 

So, I'd be curious if you measure ACTUAL MPG vs. what the computer tells you it is, how many would be just as disappointed?  Don't get me wrong, it is a nice car and I really enjoy driving it, but found that between the 10mpg less than expected, and a higher insurance premium (which was a surprise), I am not saving any money over the higher car payment.  I got rid of my dream car chasing the rainbow  of saving money every month only to find this isn't a reality.     

[StoBro2]

I track all of my fuel purchases through Fuelly. Everything is based on trip odometer readings and actual gallons delivered by the pump. I am assuming THOSE numbers are accurate, but who really knows...? My last fill up clocked in at 48.6 MPG. All of the computer generated numbers are interesting to look at but I don't use them in any official capacity.

[Recumpence]

It is obvious you are very frustrated. My mileage is off by 1.5mpg when calculated at the pump versus the car computer. I have kept track of it from the very beginning. If yours is really off by 5mpg, there is something wrong with you car computer.

 

I can tell you I am over 50mpg true mileage. There are those of us who are getting way over the epa with our cars. It does seem like some cars are just getting poor mileage no matter how hard the driver tries.

 

matt

Edited May 19, 2013 by Recumpence

[Riddley]

Some time in the future, there will be some regional C-MAX rallies. Owners can swap stories and driving tips. One of the possible benefits of such an event would be that two C-MAX owners can take a drive in the same car at the same time.

 

For those interested in improving FE:

If one of the drivers is not getting great FE and the other is, then they might swap cars and see if it has more or less to do with the car or driver. If it turns out to be mostly due to the car, that will become apparent. If it is the driver, then when both are in the same car, the one can make some suggestions to the other.  

 

Just some thoughts...

[RaPieR]

Also have to take into account variations at the pump when you fill.

 

I've been gassing up only in the morning and filling at the first notch of the pump trigger and only 2 clicks to try to keep it consistent. And still get more or less actual fuel use differences between the computer and pump. I don't know if there is any vehicle that would display the exact amount every time.

[RaPieR]

I forgot to note that I started this just recently and it seems to have made the difference and variance a little better

[wamba2000]

Trusting the computer in the Cmax is like not reconciling your checking account.....sure, the bank does not intend to double process a charge, but how would you know if it happened?  Best to track the miles driven and number of gallons and either calc by hand or use Fuelly to calc for you.

 

My experience, with my gauge/readings is that I put in more gas than the trip computer says I should have to.

[wab]

I track all of my fuel purchases through Fuelly. Everything is based on trip odometer readings and actual gallons delivered by the pump. I am assuming THOSE numbers are accurate, but who really knows...? My last fill up clocked in at 48.6 MPG. All of the computer generated numbers are interesting to look at but I don't use them in any official capacity.

 

Honda lost a class action a few yrs. ago because their odometers were adding miles = better MPG.

 

My friend had a few miles added to his warranty.

 

Note to self: Tell Don he never got 42 MPG. Your car was re-calibrated when it went in for service after the class action.

Edited May 20, 2013 by wab

[chuckles]

May I chime in on the difference between indicated gallons used vs. actual when filling up.

 

I have 5600 miles on my CMax and have kept a spreadsheet on all the info provided by the computer.  I have seen that my fill-ups have to add from 3 to 7.5% more fuel than the indicated gallons on the trip screen that I reset at every fill-up. 

 

Currently it is averaging a 4.6% error, so indicated my lifetime 44.02 mpg is actually 42.11 mpg.  Or to get 47 mpg I would have to have indicated 49.26 mpg.   I think others have complained about the error and don't know if Ford could or would do anything about.

 

Bottom line is I started at 40 mpg and have been just over 44 on my last two fill-ups.  Next goal 45.

 

 

[plus 3 golfer]

May I chime in on the difference between indicated gallons used vs. actual when filling up.

 

I have 5600 miles on my CMax and have kept a spreadsheet on all the info provided by the computer.  I have seen that my fill-ups have to add from 3 to 7.5% more fuel than the indicated gallons on the trip screen that I reset at every fill-up. 

 

Currently it is averaging a 4.6% error, so indicated my lifetime 44.02 mpg is actually 42.11 mpg.  Or to get 47 mpg I would have to have indicated 49.26 mpg.   I think others have complained about the error and don't know if Ford could or would do anything about.

 

Bottom line is I started at 40 mpg and have been just over 44 on my last two fill-ups.  Next goal 45.

My lifetime average indicated FE = 41.7 mpg at about 5400 miles and my calculated FE = 40.65.  So, that's an error of 2.6% too low using odometer miles.  Now if I factor in my odometer error of 2.1% (odometer reads lower than actual miles at 1388 miles - the last time I checked), I would have to increase my calculated FE by 2.1% to get an accurate measurements - actual miles / actual fill-up gallons.  So now my indicated FE is pretty close to my actual FE. :)  Of course as my tires continue to wear, this will change.  So, you might want to check your odometer against GPS / mileage makers for a number of miles to see if there is a significant error.

 

Although I don't record indicated fuel used with each fill-up, when I check it's usually several tenths lower than the fill-up gallons of between 11 - 12 gallons.  So, that might be on average around a 2% difference.

 

Also, on my 2009 Jetta, I could change a multiplier to adjust the indicated FE in 1 % increments up to around +-10% total correction via a scan tool. So, it may be possible to adjust the calculation via the Ford diagnostic tool. It wouldn't hurt to ask the dealer although they would probably be reluctant to do so unless the error was significant. I doubt 5% would be considered significant.  Also, in my VW I could change the distance impulse multiplier associated with the vehicle speed sensor (IIRC, around 10 different multipliers for different wheel / tire size combinations).  So, I could correct for odometer error. Of course, this would also alter the indicated speed which was why many would change the multiplier to get a better indication of true speed since the needle on the speedometer usually showed fast by 2-3 mph at a true 70 mph or to correct the speedometer for oversize tire.

[fotomoto]

The Prius computer has similar inaccuracies and is well documented.  I've also had much simpler economy readouts in an older Chevy and two Nissans and all three were optimistic.   You'll need to fill up at the same pump and at the same air and ground temps to get anything reasonably close to accurate/repeatable pump readouts.

 

As a matter of semantics, a computer can't lie; rather, it can be inaccurate or error prone.  

[SnitGTS]

Hey guys, I'm surprised to see how many people trust gas pumps over their car.  I'm not saying the car is perfect, but gas station pumps are notorious for telling you they are giving you more gas then they are so that the station can charge you more! 

 

Case in point, I live in NJ where you cannot pump your own gas.  When I was in high school I worked at a local gas station and during my two years there they got busted for just that, and if I remember right he got a $500 fine.  Granted gas was 99 cents a gallon for regular when I worked there, but if he's off a quarter gallon for every ten gallons pumped he'd make that fine up after 20,000 gallons, which again its been a while but I think we did in a week to 10 days. 

 

Maybe its different now or in other states, but that fine just doesn't seem like much of a deterrent. 

Edited May 25, 2013 by SnitGTS

[fotomoto]

"gasoline loses pressure over time, pockets of air enter the space before the dispenser's nozzle. The longer a pump sits unused, the more likely it could affect a given quantity of fuel."

 

"The data showed 721 pumps measured short, 209 of which were short by more than 6 cubic inch per five gallons, and another 127 were short by more than 12 cubic inches per 5 gallons.

Records indicate that during inspections, 366 Houston-area pumps failed for pump jumping."

 

"The same thing happened to loan officer David Fernandez when he pulled into a San Antonio Valero last December, estimating he had about half a gallon of fuel left.  "I have a 20-gallon tank so, realistically, I figure I probably should have put in 19-and-a-half," he said.  Instead?  "I pumped in over 22 gallons."

 

 

http://www.chron.com/news/houston-texas/article/Think-the-gas-pump-is-wrong-You-may-be-right-3588193.php

[joe]

Is it also possible that heat from the lithium batteries is influencing the fuel in the gas tank?  Cold makes gas contract (appears to be less gasoline in tank) while heat makes it expand (seem fuller).

[osiebosie]

DH and I have discussed this at length, and I've a post somewhere on this forum way back on this subject.  He thinks it's because of when the car is sitting still at a light or whatever.  A normal car would be still burning fuel, but the cmax doesn't.  So perhaps that explains the discrepancy on calculated fuel usage and the car's computer's fuel usage.  The cmax knows no fuel is being used while sitting still.  The old-fashioned way of figuring mileage, and I do it that way religiously and keep up with it, shows fuel is being used while sitting still.  ???  It is a theory.