[StanDiego]

As mentioned in another thread, my 30i reports my average MPG over my first 4K miles as 30.6 MPG though my actual calculated MPG is only a hair above 28 MPG. That's a greater than 10% deviation. Has anyone else observed this? FWIW I use the Road Trip app on my iPhone to record every fill up and have recorded every fill up since new.

[RavenGX]

I would think it's normal there is some deviation.

how do you know the fuel amount displayed on the gaspump is accurate?
are you using the same timeintervals on both the phoneapp and the car?

[stlcdr]

This is what I’ve observed in every vehicle I’ve owned that has a built in MPG indication. Actual is lower than indicated. I’ve not done this calculation on this car, however, but I expect it to be lower. Maybe the next few fillups I’ll record it (motorcycle weather at the moment so the Z is ‘hibernating’).

[dholaday]

I know that my speedometer is not accurate - it reads a couple of MPH higher than my GPS when I happen to check. I'm guessing that maybe the odometer might also read a little high - that might account for the differences in MPG between your car and phone

[StanDiego]

Originally Posted by RavenGX

I would think it's normal there is some deviation.

how do you know the fuel amount displayed on the gaspump is accurate?
are you using the same timeintervals on both the phoneapp and the car?

The pumps, by law, are regularly calibrated and certified. Yes, same time intervals (since taking delivery, assumed a full tank when I took delivery).

[BEAR-AvHistory]

Not sure how the car calculators the mileage but I use it as other cars also calculates the mileage & it makes for an easy car to car compare.

FWIW the EPA says this:

Premium Gasoline
26-24- 31MPG combined/city/highway
356 miles Total Range

My actual pretty much hits the highway number on the nose & range number is close. I usually have about a needles width more than 1/2 a tank when I get at either end of my trip.

I drive in Sport+ in the city so I am always under the EPA city figure.

Next run will check how much is required to refill, believe its around 6/7 gallons but have never filled right after a trip. Might be a few weeks before I can report an exact number.

The other question on actual speed. The speedo read fast by its factory setting as in the EU IIRC they can be fined if it under reads.

If you want to you can BimmerCode it to actual. Regardless the odometer reads actual. I leave mine reading fast as a speeding cushion.

[StanDiego]

Originally Posted by dholaday

I know that my speedometer is not accurate - it reads a couple of MPH higher than my GPS when I happen to check. I'm guessing that maybe the odometer might also read a little high - that might account for the differences in MPG between your car and phone

I'm not convinced of a correlation there but who knows.

I wasn't expecting the numbers to closely match, just a bit surprised the delta is greater than 10%, curious if others have the same experience.

[GerryC]

Originally Posted by dholaday

I know that my speedometer is not accurate - it reads a couple of MPH higher than my GPS when I happen to check. I'm guessing that maybe the odometer might also read a little high - that might account for the differences in MPG between your car and phone

With modern digital instruments, speed and mileage are not correlated. They are individually calibrated. Mileage is accurate with the tire size installed at delivery. Speed is displayed 3-5 mph above actual.

Bimmercode has a setting to display true speed. But BMW blocked that coding in the US some time ago. I believe it was with the F series.

[RavenGX]

Originally Posted by StanDiego

The pumps, by law, are regularly calibrated and certified. Yes, same time intervals (since taking delivery, assumed a full tank when I took delivery).

Hard to say without knowing how it's programmed, but my guess is there is most likely a difference in calculation method.

For example your app uses fuel inserted over x miles and then averages it over those 2 data points.

While the car probably uses the same data as used in the fuel economy graph in the idrive, which is most likely an average of actual fuel usage measured on a fixed time interval.

[StanDiego]

Originally Posted by RavenGX

Hard to say without knowing how it's programmed, but my guess is there is most likely a difference in calculation method.

For example your app uses fuel inserted over x miles and then averages it over those 2 data points.

While the car probably uses the same data as used in the fuel economy graph in the idrive, which is most likely an average of actual fuel usage measured on a fixed time interval.

Yes, the app uses simple math, miles since last fill-up divided by gallons of fuel. I suspect the car uses some pretty complex calculations to get an approximation but, given all of the variables, it's not able to precisely track how much fuel is actually being consumed. It's likely no ICE vehicle manufacturer is able to do that but they're all getting better at it over time.

[Bimmerfun82]

Definitely, but it depends how many times you cycle the car on and how much you use eco pro. In my experience if you turn the car on only a few times on a road trip and don't use eco pro it's fairly accurate, but if you use eco pro a lot it overestimates MPG because of coasting.

[StanDiego]

Originally Posted by Bimmerfun82

Definitely, but it depends how many times you cycle the car on and how much you use eco pro. In my experience if you turn the car on only a few times on a road trip and don't use eco pro it's fairly accurate, but if you use eco pro a lot it overestimates MPG because of coasting.

Interesting theory.

I virtually never use eco pro, defeats the purpose of why I bought it though I do use sport plus fairly often. Typically, I cycle two or three times, start-home or start-destination-home. When I run errands, I usually take the other car. I'm not dismissing the idea but it doesn't explain my delta.

[Bimmerfun82]

Originally Posted by StanDiego

Interesting theory.

I virtually never use eco pro, defeats the purpose of why I bought it though I do use sport plus fairly often. Typically, I cycle two or three times, start-home or start-destination-home. When I run errands, I usually take the other car. I'm not dismissing the idea but it doesn't explain my delta.

Interesting. Question - the fill-ups, could they be off by 0.1 or 0.2 gallons, potentially? That would make a difference of 0.3-0.4
MPG over a 140 mile drive. How accurate is the gas shutoff between gas stations?

Honestly I'm with you - feels like someone is cheating the MPG game and it's not just BMW, our Audi was 1.0 mpg off every time even on long road trips.

[StanDiego]

Originally Posted by Bimmerfun82

Interesting. Question - the fill-ups, could they be off by 0.1 or 0.2 gallons, potentially? That would make a difference of 0.3-0.4
MPG over a 140 mile drive. How accurate is the gas shutoff between gas stations?

I'm not sure on the shutoff but I would think the differences would average out over time. I never "top up" the tank. When the first shutoff occurs, I put the hose away. I'm sure some shutoffs are more sensitive than others so I never give much credence to MPG calculations for a single tankful.

I agree the fill-ups could be off by even a quarter gallon or more from one to another but suspect they ultimately average out the delta over time. A low fill-up on one occasion would typically result in a larger fill-up the next, right?

I appreciate the ideas, food for thought.

[Bimmerfun82]

Originally Posted by StanDiego

I'm not sure on the shutoff but I would think the differences would average out over time. I never "top up" the tank. When the first shutoff occurs, I put the hose away. I'm sure some shutoffs are more sensitive than others so I never give much credence to MPG calculations for a single tankful.

I agree the fill-ups could be off by even a quarter gallon or more from one to another but suspect they ultimately average out the delta over time. A low fill-up on one occasion would typically result in a larger fill-up the next, right?

I appreciate the ideas, food for thought.

Plus I've never had the car MPG be LOWER than the manual calc.

Another factor might be auto start/stop. Does the car incorporate that calculation correctly?

Here's a good article from C&D:

https://www.caranddriver.com/news/a1...how-to-fix-it/

Excerpt: "These sources of potential imprecision are real, but if you want to accurately track fuel consumption with your trip computer, you need to calibrate it [see below]. On the 335i, a service menu is accessible via an arcane procedure I found on the internet. Within that menu, I was able to change the trip-computer calibration. Changing the default value from 1000 to 1035 aligned the trip computer with reality. Sadly, no such adjustment exists to fix the BMW’s appallingly optimistic speedometer."