FFH MPG vs Fuelly MPG - which one is wrong?

[Charrua]

After logging 28 fill-ups I can say with certainty that Fuelly and the FFH do not agree. Both use the number of miles reported by the FFH, therefore the difference can only be due to the number of gallons reported by the FFH vs the number of gallons reported by the gas pump.

 

In my case Fuelly reports a lifetime of 40.4 MPG and the FFH 41.6 MPG. This averages out to more than 1/3 gallon discrepancy each time I fill up.

 

How can the FFH flow sensor be that inaccurate? Or, if you really trust Ford, how can so many gas station pumps be that inaccurate?

 

Remember, actual miles driven is out of the equation so it has nothing to do with tire pressure, size, etc.

[smr2112]

I have had the same experience and I too have been wondering how accurate the flow sensor is?

I would say that Fuelly is typically 5% or so less than what the display says. I thought that

the gas station might be ripping me off, but using different stations for all of us sort of rules

this out.

 

My lifetime says 42.4 and Fuelly at 41.3 or so. But per tank, it's more like 5% overall

difference. Fuelly says 42.3 and car says 44.7 last tank. There's the 5%. Fairly

consistent for me.

 

I think I stop filling after the second auto-shutoff, and not sure what the recommended practice

should be.

 

It may just be that the fuel-flow measurement has a +-5% margin of accuracy.

 

[Charrua]

 

It may just be that the fuel-flow measurement has a +-5% margin of accuracy.

 

 

My personal opinion is that all FFH are consistently off -3 to -5%. If some FFH are showing a Fuelly better than the FFH reported average after multiple fill-ups, that would prove me wrong.

[keithsm2]

I have had the same experience and I too have been wondering how accurate the flow sensor is?

I would say that Fuelly is typically 5% or so less than what the display says. I thought that

the gas station might be ripping me off, but using different stations for all of us sort of rules

this out.

 

My lifetime says 42.4 and Fuelly at 41.3 or so. But per tank, it's more like 5% overall

difference. Fuelly says 42.3 and car says 44.7 last tank. There's the 5%. Fairly

consistent for me.

 

I think I stop filling after the second auto-shutoff, and not sure what the recommended practice

should be.

 

It may just be that the fuel-flow measurement has a +-5% margin of accuracy.

 

Really doesnt matter where you stop filling .....as long as you are consistent......and after 10 or so fillups.....it should ALL equal out. I think the sensors in the ford are slightly off....my fuelly is always less than reported by car. if it was a fueling issue....there should be some times where the fuelly is more than reported by car..

[majorleeslow]

I've used my own tracking in excel and ffh13 trip summary matched the fill up quite accurately. this was done for the first 1k miles. I'm still gathering data for mpg after the TSB13B07. hopefully it still matches.

Edited September 3, 2013 by majorleeslow

[acdii]

The car is averaging fuel use, not really knowing how much fuel is actually used. It looks at all parameters of ICE operation, speed, distance traveled, and gives a calculation of estimated fuel used based on that. I highly doubt it uses a flow meter, it would have to have one on the return line and a volume meter to know how much fuel was delivered vs being returned to the tank. (providing the FFH uses a closed loop return system and not a dead end one.

 

Considering that the speedometer isnt accurate, that alone will throw everything else off. A 2-3 MPH variance can make a big difference. Trust but verify, thats what I use Fuelly for. The readout in my Flex is much more accurate than in the Fusion, but I attribute that to the fact its 100% ICE. Every one of my Hybrids was off at least 5% or more actual vs dash. The ICE only cars though are usually within .5 MPG.

[jeff_h]

Mine is normally about 2 MPG more on the display than it is with actual calculation, and I keep a spreadsheet that matches the fuelly entries.

 

Lifetime on the display is currently 50.2 over 22k miles, lifetime on fuelly/spreadsheet (aka the 'actual') is 48.3.

[acdii]

ON mine the lifetime, which reset when I first drove off the lot is at 44.2, Fuelly is 42.8

 

My Flex though, same thing, reset when I pulled off the lot is at 20.9, Fuelly 20.5.

[keithsm2]

ON mine the lifetime, which reset when I first drove off the lot is at 44.2, Fuelly is 42.8

 

My Flex though, same thing, reset when I pulled off the lot is at 20.9, Fuelly 20.5.

so one is 2 percent off....the other is 3 percent off.......these dang hybrids are hard to get accustomed too........a small percentage make it seem much more....... just like not getting EPA mileage missing the number by 3 percent on a 47 mpg rated car just seems different than missing it by 3 percent on a 20 mpg rated car.

[murphy]

The car doesn't need a flow meter. Fuel injectors flow fuel at a precise rate when they are enabled. Otherwise the computer would not be able to put the correct fuel amount into the cylinder based on power demands. The computer knows how long each injector was on and can calculate the fuel used from that data. It should be more accurate than a paddle wheel flow meter. Part of the problem is they appear to be truncating instead of rounding data for the displays.

 

If an attempt is made to completely fill the tank fuel is probably being sent back to the gas station pump via the vapor recovery system which means paying for gas that didn't go into the tank. There must be a vapor pocket in the fuel tank to allow the fuel to expand as it gets warmer. Otherwise as it expands it would come out the overflow and drip on the ground which, in the worst case, could result in a car fire. It will be expanding because the fuel in the underground fuel tank is around 55 degrees F. It will immediately begin warming up to the ambient temperature or even higher if the car is sitting in direct sunlight.

[hybridbear]

The car doesn't need a flow meter. Fuel injectors flow fuel at a precise rate when they are enabled. Otherwise the computer would not be able to put the correct fuel amount into the cylinder based on power demands. The computer knows how long each injector was on and can calculate the fuel used from that data. It should be more accurate than a paddle wheel flow meter. Part of the problem is they appear to be truncating instead of rounding data for the displays.

 

If an attempt is made to completely fill the tank fuel is probably being sent back to the gas station pump via the vapor recovery system which means paying for gas that didn't go into the tank. There must be a vapor pocket in the fuel tank to allow the fuel to expand as it gets warmer. Otherwise as it expands it would come out the overflow and drip on the ground which, in the worst case, could result in a car fire. It will be expanding because the fuel in the underground fuel tank is around 55 degrees F. It will immediately begin warming up to the ambient temperature or even higher if the car is sitting in direct sunlight.

In a Click & Clack column or somewhere I remember reading that some manufacturers use data from only 1 fuel injector and extrapolate that across all of them. This was the given explanation for some of the inaccuracy.

 

Considering the speedometer/odometer error I did some calculations on a spreadsheet. During our road trip we found that the odometer was off by .1 miles every 5 miles. This means that every 100 miles on the odometer is actually 102 miles of reality. This is based on sections where different states (or Canadian Provinces) will put up "odometer verification zones". If I increase the mileage of each fill up by 2% then the car and Fuelly are almost exactly in sync. After the PCM update I reset the car's Lifetime Fuel Economy so now I'll only have Fuelly for tracking my Lifetime.

[acdii]

Most interstates have mile markers, or used to, that I would verify the odometer to, but since I haven't done so in the new Fusion, I don't know just how out of sync it is. I do know based on GPS it is off by 2 MPH @ 65 MPH

[majorleeslow]

Also note that tire pressures and tread wear can also affect your odo speedo readouts. your readouts may just be normal given these variables can change just due to temperature.

Edited September 7, 2013 by majorleeslow

[Skydogz]

I am searching but cannot find this: How do you reset you lifetime mpg, my first tank only was horrible.... dealer driving?

[corncobs]

Left display under Settings there is a reset lifetime in on of sub menus.

 

http://youtu.be/HMRYC57ePyI

 

At the end of the video it shows the reset as well.

 

From a laptop magazine article:

 

The car does this by learning your driving patterns over a two to four week period. Once it understands your usual routes it will automatically activate EV+ Mode at a predetermined distance from your home and run on electric power the rest of the way. You can reset EV+ Mode by selecting Lifetime Summary Reset in the Settings menu.

Edited September 28, 2013 by corncobs