[07E92_335i]

I noticed people complaining about getting 13mpg in their 335i and were very upset and trying to figure out why. I thought there was no way that was possible, turns out I was right.

So my 2003 BMW was able to figure out how to calculate MPG, but my 2007 (all E92s and probably many more actually) do not know how to.

So what do I mean?

When you are sitting still, idling, at a stop light, etc. So 0 mph, your gas mileage is still being calculated like you are driving. So you are burning gas but going nowhere (0/x=infinity), so BMW takes this as a DRAMATIC affect on your MPG. If you average 20mpg when driving, but you decided to sit in the parking lot a few seconds, or stopped at a few red lights, your MPG is DRAMATICALLY affected, like way too much.

I reset it and went for a 8 mile drive. Averaged 24mpg when driving, I then hit a red light at 3.5 miles and timed it, 12 seconds. My MPG went from 24.3 to 12.4 in 12 seconds. By the end of the trip (8 miles) I had averaged 15.6 MPG. While instantaneous MPGs read around 24 when driving.

So my driving MPG was 24ish. But the BMW computer says it is 15.6. I don't know why they decided to start calculating it like this, and why they thought idling should be factored in so highly.

[jpirelli]

What's wrong with this? Do you somehow magically not burn gas when you're idling?

[Efthreeoh]

Sorry, 4th-grade math aside, I've kept gas records for my car since mile 3 (delivery) and record the "BC MPG" and calculate the MPG the classic way (miles/gallons). I have traced the BC MPG vs. the hand calculated MPG and the average delta between the BC calculated MPG vs. the hand calc'd is -0.298. The car knows how to calculate MPG.

And that's data for 285,000 miles worth of fill ups.

[07E92_335i]

I've decided to not be so grumpy, and have another go at it. It seems like your car is clearly doing the calculations correctly. But mine is making the idling time count way too much, meaning, it counts idling 12 seconds and using a tiny bit of few as much as driving 8 miles or so.

[cmg5461]

Originally Posted by 07E92_335i

I've decided to not be so grumpy, and have another go at it. It seems like your car is clearly doing the calculations correctly. But mine is making the idling time count way too much, meaning, it counts idling 12 seconds and using a tiny bit of few as much as driving 8 miles or so.

The mpg calculation is simply miles logged since reset vs. fuel used since reset. If your odometer reads correctly, it might be miscalculating the amount of fuel it's using.

The only way to find out is to keep a log.

Also, 0/x = 0.
lim y/x as x->0 = infinity.

[lookalikehuuh]

Originally Posted by Efthreeoh

Sorry, 4th-grade math aside, I've kept gas records for my car since mile 3 (delivery) and record the "BC MPG" and calculate the MPG the classic way (miles/gallons). I have traced the BC MPG vs. the hand calculated MPG and the average delta between the BC calculated MPG vs. the hand calc'd is -0.298. The car knows how to calculate MPG.

And that's data for 285,000 miles worth of fill ups.

Agreed, I too track mileage manually as well as reset the trip computer on every fill up. I have found my car to be within +/- .2 MPG between the car's calculation and my own.

So yes the car is calculating properly and yes sitting at a red light/parking lot/anywhere just idling will kill your MPG since at that point you are burning fuel while traveling at exactly 0MPH.

[smithf80]

I remember what you're saying from my E90.

If you reset your MPG counter, and then come to a red light, it will move itself down drastically as you sit there not moving.

Always seemed to even itself out over time though.

[JDane07]

Originally Posted by cmg5461

The mpg calculation is simply miles logged since reset vs. fuel used since reset. If your odometer reads correctly, it might be miscalculating the amount of fuel it's using.

The only way to find out is to keep a log.

Also, 0/x = 0.
lim y/x as x->0 = infinity.

Ew, limits are gross lol

[tmo24]

I'm pretty sure the Germans can math. Think about it. The car knows how much fuel is in the car from point A to point B. Then it also knows how many miles were driven regardless of how much you idle. So if you drive 100 miles and used 5 gallons, regardless of start and stop, you got 20mpg no matter how you slice it up. You can argue that it doesn't know if you stopped and refilled fuel but I'm thinking that it would know that there was more fuel added and takes that into account. Probably some sensor that records whether or not the fuel level was raised while the car was stopped and the gas lid opened. I can't call it but j wouldn't put it past them. As advanced as the other items in the vehicle are, I can't see them making a large mistake on mpg. It has always worked out in the end for me even if I try to trick the system. I can trick it to say it gave me 99 mpg if I coast down a hill for a few miles and throw it in neutral. At the end of the gas tank, I still average around 20 mph and I still go around 300 miles and I still use 15 gallons. My only exceptions are on road trips. I get better fuel economy and it is also accurate. 3rd 335i over a 7 year span and 150k miles driven all together.

[Efthreeoh]

Originally Posted by 07E92_335i

I've decided to not be so grumpy, and have another go at it. It seems like your car is clearly doing the calculations correctly. But mine is making the idling time count way too much, meaning, it counts idling 12 seconds and using a tiny bit of few as much as driving 8 miles or so.

Modern fuel injection is extremely precise. The FI system has to calculate how much fuel to inject into each cylinder in order to attain the best burn of the fuel/air mixture to minimize emissions and make power. It calculates the amount of fuel added to the cylinder by controlling the fuel pressure and duration the fuel injector is opened to allow fuel to pass. The system does not calculate fuel consumption by determining how much fuel is in the tank. The tank fuel gauge is not that accurate, nor does the car know how much fuel was added to the tank during a fill up.

So comparing the two methods of calculating fuel mileage is interesting, because the car is far more accurate than the hand method of determining MPG since the hand method depends on an accurate accounting of fuel added in bulk to the fuel tank and an accurate accounting of miles driven, both of which are suspect to variation, where as the fuel injection system is a precise digital control of dispensing the fuel into the engine. So in reality the -.0298 variation I come up with is actually telling me the hand calculation method I use is pretty accurate. What I mean by that is I drive pretty much the same route to work every day, and fill up at the same gas station and at the same fuel pump, and fill the tank until the pump automatically shuts off.

It's tough being a car geek sometimes...

[slimxb8]

I feel its pretty right on... my only beef is... lets say ypure doing 60, reset the mpg. I drive 5 miles and am sitting at 33mpg avg. I then coast 1/4 mile at 99+mpg (I have coded to do Instant mpg) and see the avg mpg go up to 35 avg. I come to a stop at the stop sign and get back up to speed with in 1/2 mile at 12mpg. But yet my avg drops to 20mpg. So I went a quarter mile more at 65+mpg and it went up a few points but one quick stop at only -20mpg for half the time and my avg drops 15mpg.
I hope im explaining it right... had a few cars do this.

[cmg5461]

Originally Posted by slimxb8

I feel its pretty right on... my only beef is... lets say ypure doing 60, reset the mpg. I drive 5 miles and am sitting at 33mpg avg. I then coast 1/4 mile at 99+mpg (I have coded to do Instant mpg) and see the avg mpg go up to 35 avg. I come to a stop at the stop sign and get back up to speed with in 1/2 mile at 12mpg. But yet my avg drops to 20mpg. So I went a quarter mile more at 65+mpg and it went up a few points but one quick stop at only -20mpg for half the time and my avg drops 15mpg.
I hope im explaining it right... had a few cars do this.

Engines have always been and always will be more efficiency at steady state. Transient (aka acceleration) fueling is less efficient. This is why city driving gets less mpg than highway driving. Try accelerating very slowly and see how your mpg reacts.

When you're at speed, really all you're doing is pushing air from the front of the car to behind the car and overcoming all of the frictional forces involved with moving. When you're accelerating, you're using more energy to store momentum in the mass of the car. Braking wastes all of that energy

[hassmaschine]

Originally Posted by smithf32

I remember what you're saying from my E90.

If you reset your MPG counter, and then come to a red light, it will move itself down drastically as you sit there not moving.

Always seemed to even itself out over time though.

that's because when you are idling you are getting ZERO miles per gallon.

this thread makes me

[motoguy128]

Ummmmm.... It's probably not the idling that using all the fuel. Its' more likely the ACCELERATION to get back up to speed.

But since you have a 335i, you're not privileged enough to have a mpg gauge and watch it pegged to the right at 12mpg even when accelerating lightly. I think it should read as low as 5mpg for maximum effect.

When idling you are getting 0mpg. But you aren't using much fuel to run to run the alternator, AC and keep the engine spinning at 600-800RPM, so idling doesn't hurt it as much as you'd think.

[smithf80]

Originally Posted by hassmaschine

that's because when you are idling you are getting ZERO miles per gallon.

this thread makes me

Yeah. Except you are not using as much gas as that math would lead you to believe but like I said it evens out.

You don't need to argue, because the new BMWs don't count idle to negatively towards mpg.

[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.

[cmg5461]

Originally Posted by motoguy128

Ummmmm.... It's probably not the idling that using all the fuel. Its' more likely the ACCELERATION to get back up to speed.

But since you have a 335i, you're not privileged enough to have a mpg gauge and watch it pegged to the right at 12mpg even when accelerating lightly. I think it should read as low as 5mpg for maximum effect.

When idling you are getting 0mpg. But you aren't using much fuel to run to run the alternator, AC and keep the engine spinning at 600-800RPM, so idling doesn't hurt it as much as you'd think.

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

[csu87]

WTF kind of thread is this?

I idle a lot, and I hand calculate my mileage and the mileage shown is within about .5mpg of the actual.

I think OP and many others are overthinking this... Go fill up your tank completely, redo your test and then go fill up your tank completely again and do the math. I bet it is going to be close to whats shown.

[mojobmw_e90]

Who cares, drive the danm car and enjoy it. If you want mpgs go by a diesal 335

[Teufel_Hunden]

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

Math hurts my brain...

[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.

[lichtgelb]

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