[jt335i]

The instantaneous MPG and the average MPG are calculated differently by the car’s CPU. I don’t know exactly how, but as what Efthreeoh mentioned they are precisely calculated by the car’s fuel injection system. On long drives at a constant speed, I’ve seen my instantaneous MPG read 30.5 while my Average MPG is 28.5 and slowly creep up. Also, the mileage range is calculated differently and does not go by the Average MPG read out. Again, the car’s fuel injection system makes a fuel consumption calculation by counting the number and duration of pulses made by the fuel injectors as they squirt gasoline into the combustion chambers of the engine. Then, the onboard computer system’s fuel economy gauge divides the distance the car travels by this estimated fuel consumption calculation. So, for example, if you had recently been travelling on the highway, and stopped to fill up, your mileage range is recalculated based on that last driving session. However, to get an overall estimated MPG average I always reset the trip odo at the previous fill up and divide the miles by the number of gallons at my next fill up. I compare that with my car’s CPU and they’re pretty close.

But, anytime you’re idling for extended periods of time, it will kill your MPG average and that’s not only at long light traffic lights. That’s why I rarely ever go to any kind of drive-thru at a restaurant, drug store, or wherever. And don’t forget there are other things such as if you’re in your car idling while waiting at a school to pick up your kids (for those who have kids), or waiting at a restaurant to pick up your “to-go” order, or at a rail road crossing, or waiting to pick someone up at wherever place, yep that’ll kill it too.

[fazman]

Originally Posted by jt335i

The instantaneous MPG and the average MPG are calculated differently by the car’s CPU. I don’t know exactly how, but as what Efthreeoh mentioned they are precisely calculated by the car’s fuel injection system. On long drives at a constant speed, I’ve seen my instantaneous MPG read 30.5 while my Average MPG is 28.5 and slowly creep up. Also, the mileage range is calculated differently and does not go by the Average MPG read out. Again, the car’s fuel injection system makes a fuel consumption calculation by counting the number and duration of pulses made by the fuel injectors as they squirt gasoline into the combustion chambers of the engine. Then, the onboard computer system’s fuel economy gauge divides the distance the car travels by this estimated fuel consumption calculation. So, for example, if you had recently been travelling on the highway, and stopped to fill up, your mileage range is recalculated based on that last driving session. However, to get an overall estimated MPG average I always reset the trip odo at the previous fill up and divide the miles by the number of gallons at my next fill up. I compare that with my car’s CPU and they’re pretty close.

But, anytime you’re idling for extended periods of time, it will kill your MPG average and that’s not only at long light traffic lights. That’s why I rarely ever go to any kind of drive-thru at a restaurant, drug store, or wherever. And don’t forget there are other things such as if you’re in your car idling while waiting at a school to pick up your kids (for those who have kids), or waiting at a restaurant to pick up your “to-go” order, or at a rail road crossing, or waiting to pick someone up at wherever place, yep that’ll kill it too.

You guys do know you can calibrate the BC MPG using the hidden menu in the kombi. I forgot which menu item it is after you "unlock" but I have tested it out with my e90 and the default value is 1000 and you have a value range from 750 to 1200-ish i believe? Been a while since I looked at it in my car.

[NoSlow5oh]

This thread makes me wonder, with all the complicated math and records, if we had a way to understand all the mpg differences from a vehicle. How would we go about it?

We could have a standard company to do all the math. Maybe an agency, with ties to protecting the enviroment.
They would take a vehicle when new and calculate the mpg's. NOT just point A to B though.
They would be able to calculate the mpg's when you have to stop and accelerate, simulating the scenario of being in a city.
They would also be able to calculate the mpg's when you are driving steady on a long trip, simulating being on a highway.
They would calculate these values and provide you BOTH driving scenarios, such as city and highway mpg's.

Why hasn't anyone thought of this?

[oneup335]

I accepted that my 335 is a gas guzzler, if I wanted to save gas I would kept my prius

[Antetokounmpo]

So you didn't just think to reset the trip meter after filling up then taking miles driven on the tank divided by gallons of gas on your next fill up?

More logical and perfectly accurate.

[smithf80]

Originally Posted by hassmaschine

it's still zero miles per gallon. New BMWs absolutely still count it towards your average, otherwise they would be overly optimistic on what your average fuel economy is.

I know, math is hard.

You can't count mpg while not going any distance. Math is hard for you I guess.

While the technicality that when at idle you are getting 0mpg, it shouldn't be making your overall drop.

If you reset your mpg counter and then go drive in a suburban area, you stop at a red light, yet while you're sitting there it takes you from 14mpg to 9 because of how it calculates it, that's not right.

Its the acceleration afterwards that makes your mileage shitty. Not your dumb logic that you do get 0mph while idling.

[JasW]

Originally Posted by Jeff@TopGearSolutions

Fill up tank at gas station until it clicks, set your trip to 0. Drive for an extended period on said tank.

Fill up tank again at same gas station and pump.

Take trip miles and divide by gallons to fill. (IE: 300 Miles / 12.5 gallons to fill) = 24 MPG.

The AVG calculated MPG is usually on par when weighted over several months without reset. However, a large change in driving habits IE 100% highway VS 80% city 20% highway will skew the results, Ive noticed.

Best to just do it the old fashion way with trip and filling up at same gas station, IMO.

When I traded in my 2008 335i for my 2013 335is, and found I was getting a good 5-6 mpg less, I did this for a while, not trusting what I was seeing on the car. The calculations lined up almost precisely with what I was seeing on the car. I just get shitty gas mileage.

[Jeff@TopGearSolutions]

Originally Posted by JasW

When I traded in my 2008 335i for my 2013 335is, and found I was getting a good 5-6 mpg less, I did this for a while, not trusting what I was seeing on the car. The calculations lined up almost precisely with what I was seeing on the car. I just get shitty gas mileage.

I believe it's accurate but only long term.

[JasW]

Originally Posted by Jeff@TopGearSolutions

I believe it's accurate but only long term.

If your driving (amount of city and highway, plus traffic) is fairly consistent day in and day out, it's accurate. Long term, I'm not so sure -- when I fill up with an E60 blend, I divide the car's miles remaining readout by the mpg readout to guesstimate how much gas is left in the tank. If I reset at each fill up, the readouts are more accurate at the next fill up than when I don't reset.

[lichtgelb]

At least we aren't driving a VW with completely bogus emissions.

[Antetokounmpo]

Originally Posted by 07E92_335i

Well seems like this thread is over, this would explain it. If it can be "calibrated" that means it can be wrong and I'm sure mine is not calibrated correctly saying it's WAY wrong.

I'm really baffled why you don't test it using the trip meter when you fill up.

[csu87]

Originally Posted by 07E92_335i

computer average MPG: 15.9 (reset at same time I started calculating by hand)
Miles driven: 266
Gallons of fuel used: 11
...doesn't seem to add up

Math IS hard I guess.

This is using the fill to full method Or just your estimated method from your original post?

[NoSlow5oh]

Originally Posted by 07E92_335i

computer average MPG: 15.9 (reset at same time I started calculating by hand)
Miles driven: 266
Gallons of fuel used: 11
...doesn't seem to add up

Math IS hard I guess.

I know on mine the trip mileage on the idrive said over 3000 miles it was using for the average mpg, and I'm pretty sure it doesn't reset with the trip meter press next to the guages because I reset that every week. I think you need to reset it in the iDrive at the same time you reset trip meter button on guages for it to work correctly.

[Dozhdbog]

Originally Posted by lichtgelb

This thread would make more sense on a hypermilling Prius forum.

Actually, I'd like to know how well people do hyper-miling their e9x's.
I've never gone full eco-warrior with it, but I have done long road trips using certain hyper-miling techniques just to see how high I could get my mpg over a tank of gas. Best I've done is 36.6 mpg.

[csu87]

Originally Posted by NoSlow5oh

I know on mine the trip mileage on the idrive said over 3000 miles it was using for the average mpg, and I'm pretty sure it doesn't reset with the trip meter press next to the guages because I reset that every week. I think you need to reset it in the iDrive at the same time you reset trip meter button on guages for it to work correctly.

That is correct

[07E92_335i]

Originally Posted by NoSlow5oh

I know on mine the trip mileage on the idrive said over 3000 miles it was using for the average mpg, and I'm pretty sure it doesn't reset with the trip meter press next to the guages because I reset that every week. I think you need to reset it in the iDrive at the same time you reset trip meter button on guages for it to work correctly.

Weird, didn't know that. I'll have to give that a try.

[lichtgelb]

Originally Posted by Dozhdbog

Actually, I'd like to know how well people do hyper-miling their e9x's.
I've never gone full eco-warrior with it, but I have done long road trips using certain hyper-miling techniques just to see how high I could get my mpg over a tank of gas. Best I've done is 36.6 mpg.

I haven't tried it with my E90 but I was able to get 21.1 in town with an Acura MDX by surge and coasting and timing green lights. The norm for that MDX was 15 mpg in town. It's possible to squeeze out good mileage like that but it's kind of an obnoxious way to drive.

[Antetokounmpo]

Originally Posted by 07E92_335i

I do, and it's completely different to the actual one. If you read the the title of the thread and the first comment, you can clearly see that this has nothing to do with the purpose of this thread.

What? No. Your logic doesn't even make sense.

I'm not going to bother reading your "math"

The computer takes miles traveled since reset/gallons of fuel used. Simple as that. Yes, the mpg will go down while idling because the car is still using fuel while idling and not moving. The whole point of miles per gallon is to determine how much fuel a vehicle uses over a certain amount of miles. Say you want to go 100 miles and figure out how much fuel you'll need. Half is city, half is highway. You will not be moving the whole time. You will stop. That is guaranteed. You take the 100 miles divided by the combined mpg to get an approximate value of how much fuel the vehicle uses.

I don't know why you don't understand it. BC MPG is accurate with +-0.2 for me usually and I sit in stop and go traffic a decent amount. And I coast in neutral when slowing down often.

[Antetokounmpo]

Originally Posted by 07E92_335i

266 miles/11 gallons does not equal 15.6mpg. I have no idea why this is hard to understand.

Maybe it would've been easier if you put that in the first post.

Obviously something was altered for mpg calculation, you're the only one having issues.

[sy2767]

I didn't take the time to read every post on the past 3 pages but if your that concerned about calculating MPG and usage maybe drive something more fuel efficient if it bothers you that much. This topic has been beat to death I'm sure, It is what it is and that's the nature of the beast. My mildly tuned 335 gets about 19-22MPG city and 29-34 hwy but my 2008 Trailblazer SS gets only 10-12 MPG city and 16-17 MPG hwy, that's the price I pay for driving a suv with a 6.0 V8 and 400hp and I accept that because of what it is and the enjoyment I get out of it. I would just focus on enjoying my awesome car if I were you.

[PhaseP]

Maybe the computer is using the fuel gauge information also in the mix, and if you were on an incline stopped it may throw it off. In the long run this will average out, because it will be running mostly on flat road and equal amounts of incline and decline on the average.

[Efthreeoh]

Originally Posted by cmg5461

Idling uses roughly 0.25-0.5 gal/hour

Idling for 1 hour = 0.5 gal (maximum)
driving 25 miles at 25mpg = 1 gal
new mpg = 25 / (1 + 0.5) = 16.7 mpg

driving 50 miles at 25mpg = 2 gal
new mpg = 50 / (2 + 0.5) = 20 mpg

Idling for 15 mins = 0.125 gal
driving 25 miles at 25mpg = 1 gal
new mpg = 25 / (1 + 0.125) = 22.2 mpg

driving 50 miles at 25mpg = 2 gal
new mpg = 50 / (2 + 0.125) = 23.5 mpg
.. and so on

It's really not as big of an impact as it seems. But yes, it's likely coming from acceleration. My instant mpg shows 6mpg as rock bottom

Considering almost every European manufacturer has a engine stop/start feature, I'd say avoiding idling is rather important to achieving a good MPG.