[Coolieman1220]

How many of you calculate your gas mileage each time you fill up?

Do you:

A) Use the BC computer to tell you what it is?
or
B) Calculate it by distance driven over fuel used.

What is the general trend for those of you who do both? I notice my BC reading is generally conservative by roughly 2 mpg. My last fill up it said 33.7 mpg and when I filled up, it was in reality 36 mpg.

[MrPig]

I use an Android app (iCar) which gives me a l/100 km reading. The trend is that the OBC is anywhere between 0.5-1.0 l/100km higher than what the app tells me. My last tank the OBC said I got 11.8L/100km while the app says it was 11.1.

[GarySL]

B, always, every fill-up.

[rbocc13]

whats the point of even figuring it out? Not like you can change it, just good to know i guess.

[Pinarello Rider]

Both. For the most part, I watch the computer, but every once in a while, I'll double check it by doing 2-3 checks using the same pump at the same gas station. I have found the computer to be off by .5mpg in either direction at any one time. I figure it's close enough for my purposes...

[dcat151]

I calculate it by method B each time. I used to do it by hand, but now I have an android app (FuelLog) that keeps track of it. I have found the OBC to be 0.5 to 1 mpg too optimistic.

[maswastage]

I always calculate it by hand, and compare it with the on board computer. In over 2 years of ownership, I've found the computer on my D underestimates my mpg by roughly 1 - 1.5 every time. The most it's every underestimated it by was about 2 mpg. My old 328i was pretty accurate in its mpg estimates, which makes me think the reason the error is higher on the D is because the mpg is so much higher.

[mike-y]

Originally Posted by rbocc13

whats the point of even figuring it out? Not like you can change it, just good to know i guess.

it is good to know. If you see changes/drops in the trend, it may indicate there is a problem somewhere.

I always use method B.

[Pinarello Rider]

Originally Posted by mike-y

it is good to know. If you see changes/drops in the trend, it may indicate there is a problem somewhere.

I always use method B.

This.

It's a great tool (among many others) to keep an eye on how your engine is performing.

[Ron1n]

I use Fuelly, period. :P

[ENINTY]

I do both for every fill up. Keep a log book in the glovebox, then transfer the data to Exel. After 158,363 miles the average difference between the BC MPG and hand-calculated method is -.216 MPG, meaning the BC is .216 over optimistic (if you can call it that). I'd bet the BC is actually more accurate considering the fluctuation in the pump for metering exactly 1 gallon at all times.

[roundle]

Both.

Also, I use "Road Trip Lite" for the iPhone.

The trip computer's about 3% optomistic.

[SeanS54]

The comp says I get about 1mpg less than my calculations. Our calculations can't be 100% accurate because you have take into account the pumps margin of error and possibly air in the tank.

[username604]

Miles over fill. I top it off from the previous drive, and reset my trip computer, my average speed, and average mpg. My mpg counter is usually accurate, but reads about 1 mpg high, depending on how much full throttle and coasting i do.

edit: oh and not that i think it really matters, but i fill at the same pump at the same shell gas station 80% of the time.

[username604]

Originally Posted by CalBear_335i

The comp says I get about 1mpg less than my calculations. Our calculations can't be 100% accurate because you have take into account the pumps margin of error and possibly air in the tank.

the on-board computer will probably be even less accurate. it never goes above 99 and never goes below 6 mpg. for some people, the acceleration margin of error outweighs the coasting margin, and so it reads high for them. for others, it's the other way around and it reads low.

but seriously, why do you guys even check your mpg anyways? who cares about 3 mpg? you guys drive bmws, you should be worrying about run-flats and battery registration? OP must be a troll. no one driving our 230-300 hp cars cares about economy!

[MrPig]

Originally Posted by davidkneiber

but seriously, why do you guys even check your mpg anyways? who cares about 3 mpg? you guys drive bmws, you should be worrying about run-flats and battery registration? OP must be a troll. no one driving our 230-300 hp cars cares about economy!

This is coming form the guy that started the coasting in N thread and used the fuel economy argument

[username604]

Originally Posted by MrPig

This is coming form the guy that started the coasting in N thread and used the fuel economy argument

yeah i was joking lol

[Clifton]

I use AccuFuel for the iPhone. It helps me manage multiple cars and lets me export the data to excel.

[Tom K.]

Originally Posted by rbocc13

whats the point of even figuring it out? Not like you can change it, just good to know i guess.

As one of my BMWs is used approx. 50% for business purposes, keeping proper records yields the maximum in tax savings. It's also useful to see if the car is running properly.

I record and reset the OBC and odometer and record miles travelled, fuel used and cost per tank. This is then transfereed to a spreadsheet.

On my '07 328i, the OBC is about 2.2% high and my '08 128i is about 1.4% optimistic.

Tom

[Coolieman1220]

Originally Posted by davidkneiber

OP must be a troll. no one driving our 230-300 hp cars cares about economy!

No troll, I drive a diesel. i got 36.5 today. computer read 34.5 i like my mileage =)

Originally Posted by MrPig

This is coming form the guy that started the coasting in N thread and used the fuel economy argument

LOL

[username604]

Originally Posted by Coolieman1220

No troll, I drive a diesel. i got 36.5 today. computer read 34.5 i like my mileage =)

don't worry man, i know your mileage; the whole forum knows your mileage; it's in half your posts
I bet you at least 50 people have gone out and purchased 335d's because of you! heck, bmw should reimburse you for that kind of advertising, or maybe they all ready are? trololol

[jvsxn]

I'm currently experimenting on how to accurately obtain the mpg. As for now, I reset the BC at every fill-up, drive from point A to point B, and then I would divide miles driven by the calculated mpg on the BC. As for double-checking at the pump, I hardly pump empty to full so I can't rely on that. Will definitely update if I find a better way to calculate gas consumption!