C-max MPG not calculated correctly

[mbedit]

My understanding is that there is a fuel gauge, which you can read how they work here . Essentially though, the gauge uses a float that is wired to a computer. If the C-Max is strictly using this to compute the MPG, then there certainly can be inaccuracy, especially if your topping off, because, as shown in the animation in that link... the float will be submerged and won't move until you consume some fuel, thus it will effectively consistently under report  the fuel consumed, which is exactly what I'm seeing.

 

However, I thought that the C-Max (and more modern cars) actually computed the volume of gas consumed using a different gauge which was part of the fuel injection system. These gauges count the duration and number of injections and can compute volume that way.

 

I'm pretty sure the float gauge is whats used to give the basic fuel gauge fire the "empty" light, but I would have thought that the other gauge would be used to compute the actual MPG.

 

Of course I'm no auto mechanic so I could be completely off base.

[jmckinley]

My car is always off about 0.5 gallons compared to the pump reading.  Last fillup, however, it was off a full 1 gallon.  Pump said 11.5, car said 10.5.

 

The error, by the way, got larger after the reprogramming in June. 

[mbedit]

Thats exactly what I'm seeing jmckinley!

[rsas]

I've had my C-Max for ~4 months and every fillup am consistently seeing 0.5-1 gallon underestimated by the computer. I get gas from several different sources so its not likely the pumps are all off. After reading this forum its clear Ford is not doing a good job in their fuel estimate. This seems like more fuel for the mpg lawsuit. This drops my mileage from low 40's to high 30's which is starting to sour me on this vehicle. I get very high scores on acceleration, braking so I dont think I can eek much more mileage out of the car.

[jmckinley]

I've had my C-Max for ~4 months and every fillup am consistently seeing 0.5-1 gallon underestimated by the computer. I get gas from several different sources so its not likely the pumps are all off. After reading this forum its clear Ford is not doing a good job in their fuel estimate. This seems like more fuel for the mpg lawsuit. This drops my mileage from low 40's to high 30's which is starting to sour me on this vehicle. I get very high scores on acceleration, braking so I dont think I can eek much more mileage out of the car.

Don't know how long you've had your car, but the "Big Fix" last summer actually made this inaccuracy worse.  In fact, it is my theory that that was the main feature of the fix.  Indicated MPG got better but actual MPG was the same as always.   

[RaPieR]

Don't know how long you've had your car, but the "Big Fix" last summer actually made this inaccuracy worse.  In fact, it is my theory that that was the main feature of the fix.  Indicated MPG got better but actual MPG was the same as always.   

 

Considering how much heat Ford received from the initial EPA ratings for the C-Max, I doubt they would be stupid enough to do something like make the trip computer more inaccurate to "hide" fuel economy.

[jmckinley]

Check it out.  Lots of other people have reported the same thing on this forum.

 

Don't underestimate their stupidity.  They released a car with high mpg estimates, knowing full well that it would not deliver 47 mpg in the customer's hands.

[DaveofDurham]

I don't think there has been any change in the spread I see between gallons the car says I have used and gallons reported by the pump as a result of any of the updates.

[ptjones]

If you want to feel better check your Odometer,  it is probably off by 1.5-2% which improves MPG's by about .5mpg's or so. :)

 

Paul

[banditb17]

This is a scary thread to necro. Now I have more data I have to collect to see if I am getting screwed or not :/

[ptjones]

This is a scary thread to necro. Now I have more data I have to collect to see if I am getting screwed or not :/

I'm confused, I just said you're getting better MPG's than you think you are, that is a good thing in my book. :)

 

Paul

[Marc Smith]

we are not alone,  the prius overreports its mpgs as well...and I'm sure my wifes saturn Vue gren line  does as well.  The vehicle knows how far its going and how fast.  and it has an idea of how much fuel its pumping.  it know the the injectors pump X amount of fuel each time they squirt.

 

It all about accuracy of the measuring equipment. and accureacy of the dispensing equipment.

 

I cna buy a digital scale on ebay for 25 bucks  that will get me within a gram.  or I can spend 2500 bucks for a scale that will get me accurate to .0001.  

 

How accurate do you want to be,  and at what cost....  Right now  I'm seeing about 5-10% difference between pumped and displayed numbers.

 

One woudl think that  ford could easily write a mild software update to compensate,  the problem.  not everyone is seeing the same delta.  so an update for me, won't work for someone else...

[banditb17]

I'm confused, I just said you're getting better MPG's than you think you are, that is a good thing in my book. :)

 

Paul

 

That's cool and all, but at the same time .5MPG gain doesn't exactly offset a 2+ MPG loss to over reporting.

 

Unless you are saying you are comparing GPS mileage to actual fuel pumped (which is probably the most accurate test, right?) which ends up in an OVERALL .5MPG increase. Then that's cool!  :rockon:

Edited October 15, 2014 by banditb17

[ptjones]

That's cool and all, but at the same time .5MPG gain doesn't exactly offset a 2+ MPG loss to over reporting.

 

Unless you are saying you are comparing GPS mileage to actual fuel pumped (which is probably the most accurate test, right?) which ends up in an OVERALL .5MPG increase. Then that's cool!  :rockon:

I use actual gas pumped and actual  GPS mileage. Best to check Odometer accuracy on a trip, my last trip on one leg I went 501.8mi. on odometer and 512.7mi. on GPS which works out to 1.021%  which = 1mpg's at 50mpg. :)

 

Paul

[fbov]

I'm curious, Paul, why do you put your faith in GPS mileage? The odo is an analog device, based on tire revolutions, so there are no interpolation errors. I'm no GPS expert, but what little I've done leads me to expect a far lower sampling rate from GPS, with attendant discrepancies to the actual distance, to the extent that you aren't traveling in a straight line. 

 

And while I've got you, please tell me how 1% of 50 isn't 0.5? This jumped out at me, so I looked at your numbers.

 

It's not  a1.021% difference, that's the ratio of the mileages. The difference is (ratio -1) = 2.1%, or 1MPG  at 50 MPG.

 

HAve fun,

Frank

[ptjones]

I'm curious, Paul, why do you put your faith in GPS mileage? The odo is an analog device, based on tire revolutions, so there are no interpolation errors. I'm no GPS expert, but what little I've done leads me to expect a far lower sampling rate from GPS, with attendant discrepancies to the actual distance, to the extent that you aren't traveling in a straight line. 

 

And while I've got you, please tell me how 1% of 50 isn't 0.5? This jumped out at me, so I looked at your numbers.

 

It's not  a1.021% difference, that's the ratio of the mileages. The difference is (ratio -1) = 2.1%, or 1MPG  at 50 MPG.

 

HAve fun,

Frank

You are right it's +2.1% to the better. BTW GPS is very accurate and you can prove it out watching mile markers and comparing the GPS distance, then the Odometer. It is very obvious. Like I mentioned New tires were +2.1 and old tires were +1.4%. I was unable to find out how often a Garmin updates per second. :)

 

Paul 

Edited October 16, 2014 by ptjones

[fbov]

Paul, I agree GPS should be very accurate in straight lines, it's the twisties I'm questioning... a route dependent error is harder to correct than a bias or calibration error. (Not that I do any correction... this ain't rocket science, or mirror making!)

 

And to get back on topic, I'm seeing an average 5% bias to one side, but not for all fill-ups. Sometimes, the odo consumption is equal or higher than gallons pumped, at all fill levels.

Fuel Fill Comparison pump v odo 141016.pdf

 

My last fill-up was only off 0.05 gal., but it was also a partial tank; BJ's offering $0.05 discount if combined with purchases, so I filled up a week early. I find it curiouis that these top-offs seem to agree better, albeit with a very small sample size.

 

Have fun,

Frank

Edited October 16, 2014 by fbov

[ptjones]

I'm curious, Paul, why do you put your faith in GPS mileage? The odo is an analog device, based on tire revolutions, so there are no interpolation errors. I'm no GPS expert, but what little I've done leads me to expect a far lower sampling rate from GPS, with attendant discrepancies to the actual distance, to the extent that you aren't traveling in a straight line. 

 

And while I've got you, please tell me how 1% of 50 isn't 0.5? This jumped out at me, so I looked at your numbers.

 

It's not  a1.021% difference, that's the ratio of the mileages. The difference is (ratio -1) = 2.1%, or 1MPG  at 50 MPG.

 

HAve fun,

Frank

I got this from Garmin:

Thank you for contacting Garmin International.

The satellites send a signal once every second.  A curvy road should not have any affect how the satellites work.   

Today's GPS receivers are extremely accurate, thanks to their parallel multi-channel design. Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. Garmin GPS receivers are accurate to within 15 meters (49 feet) 95% of the time. Generally, users will see accuracy within 5 to 10 meters (16 to 33 feet) under normal conditions. 

Factors that can degrade the GPS signal, which can affect accuracy, include the following:

1) Ionosphere and troposphere delays: The GPS signal slows as it passes through the atmosphere.

2) Signal multipath: The GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver.

3) Orbital errors: Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.

4) Poor satellite visibility: The more satellites a GPS receiver can "see", the better the accuracy. GPS units typically will not work indoors, underwater, or underground.

 

Hopefully this clears up the GPS accuracy question. :)

 

Paul

[fbov]

I did some quick calculations, and unless you drive in circles, the integration sampling error's negligible. Were you to drive around a 100 foot circle at 22 mph all the time, the error approaches -2%. And it's a bias toward shorter distances, but only in tight, fast turns.

 

Conversely, 60 mph is 88 ft/sec, so a 16-33 foot accuracy isn't too good... until you integrate over a lot of seconds. This should be zero-valued, on average, and get smaller as you driver farther.

 

It's an academic issue, but academics are fun for some of us... makes me think funny!

Frank

[ptjones]

I did some quick calculations, and unless you drive in circles, the integration sampling error's negligible. Were you to drive around a 100 foot circle at 22 mph all the time, the error approaches -2%. And it's a bias toward shorter distances, but only in tight, fast turns.

 

Conversely, 60 mph is 88 ft/sec, so a 16-33 foot accuracy isn't too good... until you integrate over a lot of seconds. This should be zero-valued, on average, and get smaller as you driver farther.

 

It's an academic issue, but academics are fun for some of us... makes me think funny!

Frank

The problem as I see it is that the GPS says it's 2% farther. ;)

 

Paul

[plus 3 golfer]

The last time I checked my Garmin GPS miles with mile markers and the odometer was on my  2041 mile trip from AZ to PA (Mapquest door to door miles) .  GPS was virtually spot on with mile markers over 100s of miles (within tenths of a mile).  Odometer was always low by about a mile or more.  GPS showed door to door miles at 2048 which included 8 stops (fuel, food and lodging) which likely virtually makes up the 7 miles difference between Mapquest and GPS miles as I'd say we never were more than 1/2 mile from the interstates.   My odometer showd a difference door to door of 2021 miles.   So, the odometer likely read low by as much as 27 miles over 2048 miles or 1.3% low.  

 

Miles on my odometer at the PA trip end of was about 22 k. The first time I checked the odometer against mile markers and GPS was with less than 1000 miles on the car for short distances of around 100 miles or less on local interstates.  The odometer showed low by 2.1%.  So, I guess tire wear likely reduced the error on the PA trip over the first check as wheel revolutions per ACTUAL mile traveled would increase as tires wear and thus the indicated odometer miles when converting wheel revolutions to miles would move closer to the ACTUAL miles traveled.   My tire pressure should have been about the same for both trip and test - about 45 psi.

 

I've had several different Garmin GPS in the last 7-8 years and the miles traveled as indicated on the Garmin is always very close to the interstate mile marker mileages.  Actually, here's wiki's answer to GPS vs Odometer which I have validated many, many times. :)

 

Accuracy[edit]

Most odometers work by counting wheel rotations and assume that the distance traveled is the number of wheel rotations times the tire circumference, which is a standard tire diameter times pi (3.1416). If nonstandard or severely worn or underinflated tires are used then this will cause some error in the odometer. The formula is (actual distance traveled) = ( (final odometer reading) - (initial odometer reading) ) * (actual tire diameter) / (standard tire diameter). It is common for odometers to be off by several percent. Odometer errors are typically proportional to speedometer errors.

GPS used as odometer[edit]

Recently, exercise enthusiasts have observed that an advanced Global Positioning System receiver (GPSr) with an odometer mode serves as a very accurate pedometer for outdoor activities. While not truly counting steps (no pendulum is involved) an advanced GPS odometer can accurately reveal the distance traveled to within 1/100 of a mile (depending on the model, perhaps 1/1000 of a mile). 1/1000 of a mile is approximately the distance of a single pace or 2 steps. Precise metric odometers have a precision of 1/100 or 1/1000 km, 10 or 1 metre(s) respectively.