Car Fuel Mileage vs. Calculator Fuel Mileage

[osiebosie]

I'm seeing a consistant 2-3 mileage difference in the mileage shown on the car at fill-ups vs. figuring it out on my calculator.  The difference is to the car's advantage.

 

DH and I have discussed this often.  He says it's because when we are stopped at a light, we are using NO fuel, vs. a normal car, using fuel but driving zero miles.  I don't know. 

 

Is he right?

 

Thanks.

[Tdefny]

My Prius has the same discrepency. I guess they figure that an owner looking for bragging rights will be too lazy to question the gauge.

 

It is a strategy that could backfire though, because early adopters like on this forum will look at everything.

Edited December 15, 2012 by Tdefny

[mrobinso8]

I think it is a Ford thing to make it look better than it is.  If I do a compare after one tank or let it go 3 or 4 tanks on a trip before comparing it always reads 2 mpg to the advantage of the car.  This is on my 2011 Ford Fusion.  The one tank on my wife's C-Max was 2 mpg to the advantage of the car.  It is like the programmer of the car computer wrote the code to add 2 to the car's computed mpg.  My old Honda Accord was accurate to the 10th of a mile per gallon which would be to rounding errors at the pump.  Also, my Audi A4 was also accurate on it's trip computer.  Disappointing.  No reason for this except to make mileage look better to very trusting people. 

[MSmith1915]

I think there are just to many variables when one (and many different owners) fills up their car in general and their C-Max in particular.  Is the car exactly the same angle/level for all the fill ups?  Does the gas pump shut off exactly the same time for all the pumps across the U.S.?  Do you give it a couple of extra clicks after it shuts off?  Are all the gas pumps in the U.S. calibrated exactly the same?  And probably many more variables.  Even if you use the same station, same pump, temperature and barometric pressure will affect the fuel density and hence how much gas gets pumped.  As an experiment, I think I will only fill up to what the car trip display says I used.  Probably do this for a couple of months and then fill up once to when it clicks off and see how much of a difference there is.  Will let you all know how it works out. 

[Jus-A-CMax]

Well, thats why most of us use fuelly.com to track actual odo miles with how many gallons we fuel up. I agree with MSmith1915, theres too many factors here. I've had 1 fuel up only but thus far it gells. Also, as usual if there is suspicion about the mileage code, someone will inevitable hack the code and expose, for example ford, GM, toyota or whoever, now wouldn't that be a major, major scandal worthy of a haxor?

[darrelld]

I think there are just to many variables when one (and many different owners) fills up their car in general and their C-Max in particular. Is the car exactly the same angle/level for all the fill ups? Does the gas pump shut off exactly the same time for all the pumps across the U.S.? Do you give it a couple of extra clicks after it shuts off? Are all the gas pumps in the U.S. calibrated exactly the same? And probably many more variables. Even if you use the same station, same pump, temperature and barometric pressure will affect the fuel density and hence how much gas gets pumped. As an experiment, I think I will only fill up to what the car trip display says I used. Probably do this for a couple of months and then fill up once to when it clicks off and see how much of a difference there is. Will let you all know how it works out.

[darrelld]

I think there are just to many variables when one (and many different owners) fills up their car in general and their C-Max in particular. Is the car exactly the same angle/level for all the fill ups? Does the gas pump shut off exactly the same time for all the pumps across the U.S.? Do you give it a couple of extra clicks after it shuts off? Are all the gas pumps in the U.S. calibrated exactly the same? And probably many more variables. Even if you use the same station, same pump, temperature and barometric pressure will affect the fuel density and hence how much gas gets pumped. As an experiment, I think I will only fill up to what the car trip display says I used. Probably do this for a couple of months and then fill up once to when it clicks off and see how much of a difference there is. Will let you all know how it works out.

This IPS forum code is the worst I have seen. Unintended double post, no delete, non mobile clients default to mobile once you log in.

 

Back on topic, I see the same mileage differences in my Passat. Usually 1-2 mpg higher on the dash than the Fuelly calcs.

Edited December 15, 2012 by darrelld

[Bob999]

I have had cars with mileage computers for 25+ years and most computed mileage higher than I got by using the odometer and actual quantity of fuel added-- 2 mpg was a typical discrepency.

 

The exceptions were a 2009 Enclave and, I think, the CMAX.

 

I say I think on the CMAX because I only have 600 miles on it and that just isn't enough experience to get past the variation in the fill that will average out over time but can make a significant difference on a car getting 40+ mpg and a single fill.

[valkraider]

In Oregon and NJ it is against the law to pump our own gas, so we are at the whim of the attendant. It's hard to get a consistent fill up every single time...

[osiebosie]

Okay, so I am not the only one noticing that.  Good.  However, I fill up at the same gas station usually, never top off, it's very level.  The variable would be weather and filling up other places when traveling.  But it's always a couple mpgs different.

 

So no one thinks it has anything to do with idling on EV at stops? 

 

Thanks everyone.

[nsteblay]

Too bad there isn't an accurate sensor that measures the amount of fuel left in the tank.  I reset my trip meter this last fill so I can compare accurately.  I haven't found a large discrepancy yet.

 

I know in my Honda CRV if I idle at stop it does impact my MPG.  Last summer I sat with the car running while waiting at a horse show (hot day needed air conditioning).  I watched my MPG average drop constantly over the hour I waited.  

 

I would imagine it is the same with the CMax.  The fact that the gas engine doesn't run at stops is a big gas saver.

[AgentCMAX]

Too bad there isn't an accurate sensor that measures the amount of fuel left in the tank.

 

No kidding!  I just filled up my tank again yesterday and there was a huge discrepancy in the consumed gas reported by C-Max vs. the pump.

 

C-Max reported: 10.46 gallons, 462.8 miles

Actual pumped: 11.77 gallons, 462.8 miles (from odometer reading)

 

So it's keeping track of the miles just fine.  But the gallons of gas consumed is just way off.  And I noticed that the longer I wait to refill (i.e. use up more gas per fill-ups), the more inaccurate it gets.  I've only had four fill-ups so I don't know if that's a trend or just a fluke.  However, the last two times, I filled up at the same gas station using the same pump and the same technique (i.e. just set it and let it automatically shut off the pump w/o pumping more manually after the shut-off).  Temperature did cool off some, so I'm not sure how big a deal that is.

[Jus-A-CMax]

Watching this thread, I am in the same boat with this contrasting MPGs with fuelly and CMax reporting. I do keep Trip 1 reset after each fuel up so I might take a gander when I have time as well.

[AgentCMAX]

He says it's because when we are stopped at a light, we are using NO fuel, vs. a normal car, using fuel but driving zero miles.

I don't understand this statement. Could some one please elaborate on this. Is he saying because we sit still with engine off it is not using any gas so it calculates less amount of gas consumed?

 

That can't be right since doing that may result in a tank with no gas...and that's a hazard.

[AgentCMAX]

What is the proper way to fill up? It seems to me topping off is the more accurate method since you know exactly where the gas tops off and can easily determine how much fuel it takes to top off the next time since you know it is the same level each time. If you don't top off then you are at the mercy of the pump nozzle stopping (which may not be operating correctly).

[darrelld]

I don't understand this statement. Could some one please elaborate on this. Is he saying because we sit still with engine off it is not using any gas so it calculates less amount of gas consumed?

 

That can't be right since doing that may result in a tank with no gas...and that's a hazard.

Your C-Max has two ways to move, a gas engine and electric motor. If your battery has enough charge the gas engine will shut off, using no gas, and the electric motor will move the C-Max.

 

With the gas engine off its consuming no fuel and the C-Max fuel gauge registers correctly that no gas is being consumed.

 

 

Edited December 19, 2012 by darrelld

[darrelld]

What is the proper way to fill up? It seems to me topping off is the more accurate method since you know exactly where the gas tops off and can easily determine how much fuel it takes to top off the next time since you know it is the same level each time. If you don't top off then you are at the mercy of the pump nozzle stopping (which may not be operating correctly).

I have noticed the fill inconsistency that I believe is related to capless fill inlet. I reset the Trip 1 with every fill and make a note of how much fuel has been consumed since the last fillup. Next if the pump shuts off before the gallons consumed is below what the trip meter reads I continue to squeeze the pump handle until the numbers are close. Never has the gas over filled using this method.

 

As always your results may vary.

 

 

 

[AgentCMAX]

Your C-Max has two ways to move, a gas engine and electric motor. If your battery has enough charge the gas engine will shut off, using no gas, and the electric motor will move the C-Max.

 

With the gas engine off its consuming no fuel and the C-Max fuel gauge registers correctly that no gas is being consumed.

 

 

I understand how the gas engine and electric motor behaves.  What I don't understand about the comment is how it is supposed to explain the difference in the reported mileage?  What difference does it make if the mileage is based on EV or not?  You simply tally up total miles driven and divide that by total gallons consumed.  That gives you the avg. MPG value.  Unless you are trying to say the C-Max calculates the mileage for gas engine operation separately.  But if that's the case, then it's even off by a much larger margin. 

 

I'm sorry if I'm not making any sense.  I'm just not clear on how the mileage can be so way off.  Based on my experience, the C-Max appears to be way off on its tracking of consumed gas.  However, I'm not sure if my method of filling-up is correct either (even though I'm using the same pump from the same station).  So that in itself may affect the outcome.

[AgentCMAX]

I have noticed the fill inconsistency that I believe is related to capless fill inlet. I reset the Trip 1 with every fill and make a note of how much fuel has been consumed since the last fillup. Next if the pump shuts off before the gallons consumed is below what the trip meter reads I continue to squeeze the pump handle until the numbers are close. Never has the gas over filled using this method.

 

As always your results may vary.

 

 

 

Yes, but this approach still doesn't accomodate for the possible flaw in the way the C-Max determines the amount of consumed gas.  For my situation, it reported 10.46 gallons.  However, the actual filled amount was 11.77 gallons.  I can see how this method to fill a tank will address the situation of filling under the reported amount.  But how will that resolve the situation where the reported consumed gallons is actually less than what was filled? 

 

That's why I'm thinking topping off is the more accurate way since you always know the amount of gas is the same in the tank because you are making sure of it being topped off.  (I'm not recommending this by the way...just want to analyze how filling-up methods can affect actual mileage calculations).

Edited December 19, 2012 by AgentCMAX

[MikeB]

That's why I'm thinking topping off is the more accurate way since you always know the amount of gas is the same in the tank because you are making sure of it being topped off.

 

 I think the reverse is true: topping off generates less accuracy. If you insert the gas nozzle the same amount each time, and rely on the automatic shutoff, you have a pretty reliable mark for 'full', which is when the gas reaches about 6" (?) below the top of the inlet. If you top off, then you are somewhere between that 6" mark and the actual top, but you don't know how far between them you actually are. Of course, you have to fill slowly enough that you don't trigger the shutoff too early due to foaming or turbulence effects, but that's going to be the same in either case.

 

But there's possible more interesting things happening here: at least on my Energi, I think the fuel system is pressurized when driving. And it's also possible that the fuel tank isn't rigid, but a flexible bladder within a shell. Both of these could make a difference in our fuel measurements, either as it's consumed or as it's added.

[From Speed6ing 2 C-Maxing]

Has anyone checked to see how far off the odometer and speedometer are? I don't have an empty enough stretch of highway to test on.

So, an indicated 60mph vs scangauge to see how far off the speedo is.

Then a scangauge 60mph vs gps to see how far off the car's computer is.

And then, let's say for example indicated speed is same as that reported by scangauge (meaning that the ODO would be accurate as far as the car knows), but like with a lot of cars out there, it is actually over reporting, (indicated 60mph = 58mph according to GPS), then driving 600 miles and consuming 12gallons would actually be 580 miles with that same 12gallons which would mean 48.3 mpg instead of 50mpg.

 

Of course i doubt the computer is smart enough to do this, but if this was the case and we're actually getting more real world results, wouldn't that be the goal in the long run?

[AgentCMAX]

If you top off, then you are somewhere between that 6" mark and the actual top, but you don't know how far between them you actually are. Of course, you have to fill slowly enough that you don't trigger the shutoff too early due to foaming or turbulence effects, but that's going to be the same in either case.

 

Actually, I was thinking of topping it off until you can see the gas at the inlet or put some sort of flotation device with a stick in it so that you can more accurately measure it.  As soon as the stick hits the tip of the inlet, you know it's at the same level and you pull out the device.

 

But there's possible more interesting things happening here: at least on my Energi, I think the fuel system is pressurized when driving. And it's also possible that the fuel tank isn't rigid, but a flexible bladder within a shell. Both of these could make a difference in our fuel measurements, either as it's consumed or as it's added.

 

Interesting.  I can see this as helping in the delivery of the gas but I fail to see how this design would allow for accurate measurement of the gas level.  I guess if you pressurized from the top of the tank downwards so that the fluid is always at the bottom then I guess that would work.  But this would seem to be a more expensive solution...would they not want a system that costs less?

[AgentCMAX]

Has anyone checked to see how far off the odometer and speedometer are? I don't have an empty enough stretch of highway to test on. So, an indicated 60mph vs scangauge to see how far off the speedo is. Then a scangauge 60mph vs gps to see how far off the car's computer is. And then, let's say for example indicated speed is same as that reported by scangauge (meaning that the ODO would be accurate as far as the car knows), but like with a lot of cars out there, it is actually over reporting, (indicated 60mph = 58mph according to GPS), then driving 600 miles and consuming 12gallons would actually be 580 miles with that same 12gallons which would mean 48.3 mpg instead of 50mpg. Of course i doubt the computer is smart enough to do this, but if this was the case and we're actually getting more real world results, wouldn't that be the goal in the long run?

 

I don't have a Scangauge.  But I've measured my C-Max's speed with an external TomTom GPS device and it's within 0.5 to 1 mph at 60-65mph as well as at 35mph.

[MikeB]

But this would seem to be a more expensive solution...would they not want a system that costs less?

The Prius has a soft gas tank, I've seen it described as a rubber bladder. I think the biggest reason for using it is safety, there's less explosive gasoline vapor floating around, and it probably doesn't crack open after a crash like a rigid tank can. The fancy capless filler valve and the pressurization may also have an emissions purpose, keeping gasoline vapor from escaping is a really good thing to do, and I think California has requirements around that. I'm sure Ford wanted to make sure the Energi could use the California HOV lanes, which it can, but that means meeting a higher standard than the average car.

[Fits2at]

On my first fill up after the dealers tank, C-Max was reporting 38.5 mpg. When I did the actual math it came to 40.1 mpg. Will keep track as I do more fill ups.