Do you trust the onboard MPG meter?

How trustworthy is the onboard MPG display?
Do you trust the onboard MPG numbers without questioning?


Based on my experience, my manual calculations always come up short compared to the onboard MPG values. So i decided i could trust it no more.

I do quite a lot of long distance travel. This is what i always do to get my manual calculations.

1. I fill the tank till the pump/nozzle clicks;
2. I reset the trip meter to 0 and reset onboard MPG meter as well;
3. I would travel 250+ miles and another 250+ miles back home;
4. I go to the same diesel pump at the same gas station for fill up;
5. I fill the tank till the nozzle clicks;
6. MPG calculated = Y/X (Y from the trip meter; X from diesel pump display)

I repeat the same steps every time i take these trips.
my calculated MPG is always less than the onboard MPG display.

About 5mpg less on average. So if the computer displays 55mpg, my calculation would be something like 49, 50, or 51mpg.

So i decided i could not trust the computer. Previously i owned a 2011 MINI hardtop turbo which had a onboard MPG meter too, same thing, my manual calculations would always fall short when compared to its onboard displayed values. And before that car, i owned a 2010 MINI hardtop non-turbo, also the same case.

So, is it me who may have miscalculated?
or, is it the auto makers who may have tweaked the computer to display a higher value when it comes to MPG?


Either way, however, i enjoy the 50+ mpg fuel economy, definitely more so when compared to mid 30s mpg by the MINI turbo (which demanded premium gasoline).

I own a 2015 Golf TDI with manual tran. I traded my MINI for it spring 2015. I should've done it sooner. I was tired of paying for premium, the stiff ride even after i replaced those run flat tires with comfortable regular Yokohomas and the lack of interior space. However the MINI was a fun zippy car to drive, but i don't miss it.

So what is your experience like? specifically with the onboard MPG gauge.
Please share it with me, thanks.

mine is older than yours but I have seen some say theirs is way off and others say its right on. I have checked mine on a not stop 1000 mile trip, my MFD reported 50.1 and my calculated came out 49.9. on way home several days later it was that close again.

varies.

Do I trust the mpg on the dash? Yeah, I trust it to show me an calculated estimate of my mileage. However, I also know that the accuracy of that calculation can vary a lot. I have changed tire sizes and had to play with my instrument panel to make my speedometer accurate. Tire size, tire pressure, temperature, terrain, winds, and other factors can effect accuracy. It is just an estimate. It is more accurate in some cars than in others and it can vary even in the same car. In my car the total (tank) average is usually within 1 or 2 mpg. However, I have noticed that the miles to empty display can vary widely depending on driving conditions. I was once driving downhill (for miles) and it said I had over 40 miles to empty. I got to the bottom of the valley and started going back up hill and within a quarter mile it went to 5 miles to empty. The calculation is based on your current mileage - if that changes, so does the calculation.

Have Fun!

Don

the values can be modified with vcds.

When comparing the calculated figures with your display, one needs to keep in mind that the display figure is an average fuel consumption over the past two hours (at least that's the case with 2010 models). When I start the engine and take off, the first figure that shows on the display after driving few hundred meters is usually about 13 or so mpg. In a few seconds, it is up to 18, and it continues climbing for some time, until a "terminal" value is reached (under steady conditions). The time it takes to reach the terminal value depends on many factors, such as ambient temperature, humidity, DPF regen cycles, etc. If you drive longer than two hours, the data from the beginning of your trip (when you accelerated, warmed up the engine, etc.) is erased and replaced with later, more favorable data, when the engine was warmed up, and you were driving at a steady speed. So, for instance, if your average fuel economy after the first one hour was 50 mpg, then 55 mpg after two hours, and 60 mpg after four hours of driving when you arrived at your destination, it would be a mistake to expect that you averaged 60 mpg for the whole trip. The 60 mpg figure applies only to the last two hours of driving, but your fuel economy was much worse earlier in the trip, so when you calculate your actual fuel economy by dividing miles traveled and gallons used, you will always see lower figure than what the display showed. Anyways, this is how the fuel economy gauge works in my 2010 Jetta. If the on-board computer in your 2015 Golf tells you what your fuel economy was, no matter how long your trip was, then disregard my rant. I do, however, have one suggestion for you how to improve accuracy of your calculated value: when you fill up, try to get the fuel all the way up to the fuel cap. I know, one isn't supposed to do that, but this gives me the exact same volume in the fuel tank. Relying on that one click will result in varying quantity of fuel in your tank, depending on the sensitivity of the shut-off mechanism. For that reason, I only fill up at service stations that have nozzles where you can regulate the fuel flow with the trigger.

soot1 said:

When comparing the calculated figures with your display, one needs to keep in mind that the display figure is an average fuel consumption over the past two hours (at least that's the case with 2010 models).

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No..... the trip MPG is average since the last time the car was shut down over 2 hours. The total MPG average resets itself every 2000 miles. the distance to empty uses MPG average over the last 18-20 miles.

There are some software differences and Dist To Empty resets every 1000 miles on some cars. the 18-20 miles is a guess watching as I'm driving on very long up or down grades. (keeps my mind working and alert on long drives).

But there might be changes between 2010 and 2015

MKVIIs have a:
"since start" which resets only if the car has been turned off for at least a couple of hours
"since refuel" resets after fueling up
"extended period" may reset at 10k intervals..I can't recall exactly, but I leave this alone and compare with fuelly over a longer period of time.

As for the MFI calculations, I've been getting 7-8% worse than indicated. For example, last tank showed 50.3, actual was 46mpg. I'll get around to playing with VCDS and correcting this weekend.

Mine is about 7% optimistic. I tracked consumption using fuel receipts and odometer over about 3000 miles - twice actually, so the 7% is a pretty solid number. I don't have VCDS, but it's a quick and easy mental calculation once you have quantified the inaccuracy.

Last summer my display was 2mpg's higher than hand calculated. This winter however it has been a couple of mpg's lower than hand calculated...strange!

Thanks for sharing your experiences. I guess i am not the only one with inaccurate onboard MPG readings.

I suspect the onboard computer has been tweaked to read higher as a marketing tactic to attract more buyers. Given the diesel scandal by VW, i shouldn't be surprised. VW can't be the only automaker to do so.

I think one of the best ways to "force" automakers to design, engineer and produce better products is for the consumers to more educated about these products and at the same time remain skeptical, until data would actually support claims.

saganian1 said:

I suspect the onboard computer has been tweaked to read higher as a marketing tactic to attract more buyers.

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Given the reported variations in relative accuracy I think it's been tweaked to avoid anyone getting a lower than actual reading and thus being disappointed with the car after buying it (that's a fine distinction I know) - plenty of people report theirs to be <1 mpg off. Seems to be a range from nearly dead-on to 7-8% optimistic.

I pulled some mpg out of my display via VCDS, I want to say around 10%. Since then, I would say my hand calc's are within ~1% of what the long-term average displays. That's about as good as you can expect.

ShawnShady said:

the values can be modified with vcds.

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And it's very easy to do, with accurate results, if you're willing to tweak the VCDS adjustment a few times.
There is no way a manufacturer can program a display to accurately predict mpg for every driver, driving style, conditions, etc. It's kind of like expecting a "one-size-fits-all" garment to fit everyone perfectly.

I don't believe anything Heidi tells me.

There's trust, and then there is TRUST. I trust it to be somewhere in the ballpark, but I would not TRUST my life on it being accurate, since my fuelly / spreadsheet are still different than the car's instant read MPG.

I used to religiously calculate and log my mileage, not so religious lately. Also had a more consistent and longer commute.

It would read plus or minus 1 to 1.5 mpg of the calculated tank mileage, so I'm pretty happy.

akjdouglass said:

There is no way a manufacturer can program a display to accurately predict mpg for every driver, driving style, conditions, etc.

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I could be wrong, but I'm assuming the computer uses injector pulse width and frequency to calculate consumption - and if that is the case then the variations in accuracy would have nothing to do with how the car is driven. No two powertrains are going to be perfectly identical in terms of mechanical performance and manufacturing tolerances, which is likely what accounts for the variations.

scooperhsd said:

There's trust, and then there is TRUST. I trust it to be somewhere in the ballpark, but I would not TRUST my life on it being accurate, since my fuelly / spreadsheet are still different than the car's instant read MPG.

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I too would not trust it with my being

Thinking my car is a little optimistic in nature

But need more fill ups and miles to get a better handle on it.

Maybe-possibly-probably VW is messing-fibbing-lying-cheating with new owners just like they messed with CARB-EPA

hybridkiller said:

I could be wrong, but I'm assuming the computer uses injector pulse width and frequency to calculate consumption - and if that is the case then the variations in accuracy would have nothing to do with how the car is driven. No two powertrains are going to be perfectly identical in terms of mechanical performance and manufacturing tolerances, which is likely what accounts for the variations.

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I'd say you're partially correct, but I wouldn't think it would be so simple that variations from car to car can be attributed to mere differences in manufacturing (although it probably is one factor).
Regardless, it's a simple adjustment with VCDS if "close" is not close enough.

DriverJon said:

I used to religiously calculate and log my mileage, not so religious lately. Also had a more consistent and longer commute.

It would read plus or minus 1 to 1.5 mpg of the calculated tank mileage, so I'm pretty happy.

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Very happy to read your post

I drive so few miles that religious fill-ups are out of my area. So far.

I am showing unreal MPG on the last 200 mile trip to Mom's house.

to wit 60.1 MPG ~`~` 90% high way. On cruz as much as possible.
Driving 10-20 MPH over the MINIMUM speed. Lots of construction.

Heavy side winds both ways

I've been tracking hand-calc vs. MFD readout for a while now. Here is a snapshot from May-October. You can see that I don't take particular care to fill to exactly the same spot in the tank every time. I just go to the auto-stop on the pump then go one more stop/click. So every tank is subject to the variation in the 3-4 different stations that i go to. Over time, this averages out though.



So, my computer-displayed mpg over this time period was 48.2 mpg, and the hand calculated average was 44.9. That means my computer is reading about 7% higher than the hand-calculation.

andyrooski17 said:

.....my computer-displayed mpg over this time period was 48.2 mpg, and the hand calculated average was 44.9. That means my computer is reading about 7% higher than the hand-calculation.

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.....this pretty much sums up my 33k+ fuel economy profile...............

ez sends

All the trip computers, gas or diesel, that I have had have overestimated the MPG. I don't think the computer really knows how much fuel is in the tank.

jerryofva said:

All the trip computers, gas or diesel, that I have had have overestimated the MPG. I don't think the computer really knows how much fuel is in the tank.

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I don't disagree, but I'll just point out that MFI/MFD displayed mpg isn't based on the fuel level sender (which is only used for the fuel gauge and to estimate range)

There is no dedicated sensor measuring actual fuel flow, there's no need for one - the computer controls injector pulse width and frequency, and volume per second (or whatever unit of time) for any particular injector type is going to be a known finite number. Simple arithmetic does the rest.

The reason they all seem to be on the optimistic side is that (what appears to be) about a +/- 4% margin of error has been intentionally offset to reflect a +8%/-0% margin (or as close as they can get).

And my MFD has been 2>3 percent off, from day one.
The screen reads higher than actual hand calculating.
I've kept a full spreadsheet from the day I drove it
off the lot.

Bringing up this old thread because I noticed something interesting.
My MFD mileage has been 6% optimistic since I bouth the car but since I adjusted the torsion value (was way off at -3.9) I noticed that the reported mileage was exactly the same as the calculated!
Just a coincidence?

...agree with OP: the MFI is high - difference from calc'd MPG averages 3 mpg over 45,000+ miles...................


.................................................(this JSW - from a fuel efficiency, capacity, features like VTEX, 6M plus low key good design/looks - will be difficult to replace.

Can't really complain about the $$$$ VW is offering thought.........................



Happy Thanksgiving 2016




ez

wow, i havent been check this thread. it seems our experiences vary.

I am really happy about the mpg, despite the onboard display value is a little inflated.

I am gonna get a set a 15" wheels, narrower tires and that should help improve mpg further i hope. And i am gonna take the golf to dealer for the diesel modification fix.

thanks everybody for sharing, appreciate the inputs

Fuelly average is best estimate-BUT-do you trust the PUMP where you filled up!!?

It was slightly optimistic in my 2014 JSW (1-2mpg higher than actual), and a little more accurate on my 2013 Passat.

I think that long term records help establish your baseline (like Fuelly). Also, after 25-30 tanks, you get used to seeing the number of miles when you cross the half tank mark (photo below), or when the fuel-up warning light turns on. These help me estimate how it's doing on the current tankful. I think this is more informative than the number of gallons filled, because I've seen variability in the pumps I fill at! I've been ripped off by at least a gallon, and other times I know I could have put more in (only 14 gallons indicated when I knew I could fit 15). Another reason to be skeptical about single tank calculation.

This is from the Passat. It was the best MPG performer I've EVER owned. Calculation at the pump when I filled up turned out to be about 57mpg. I was on the way to a 900mile tank, easy. I filled at about 760 miles since I found a good price. This mpg was with four people and luggage, driving 60-80 on highway through windy mountain passes through Chama, and up in the Rio Grande Valley from NM to CO, then back down through Cimarron to Angel Fire (another mountain pass). Impressive tankful.